I am always confused whether I should fix/keep or change the current
That is a very difficult question, one which much stronger players, grandmasters even, often struggle with. So, it's not possible to give you a concrete answer which you can take away and always make the right decision. The best I can do is give some general guidelines which will get you thinking in the right direction. At the end of the day there is no real substitute for the vastly greater experience and hard work which top players have put in.
The key thing about changing or keeping the pawn structure is that it is a dynamic thing. If you change the pawn structure in some way, either by advancing a pawn or capturing a pawn (needn't be with one of your pawns) how does it change the dynamism of the pawns for each side and, often more important, how does it change the dynamism of the pieces for each side?
I'm going to give some examples but it can't be comprehensive because it is really the subject for a whole book which I am not good enough to write.
1) Passed Pawns
We all know the rule "passed pawns must be pushed". The dynamics of the position for me as white are very different if I have a passed pawn on a7 than on a2. In the first case my opponent will be sweating, in the second case he will be calm. Of course, make sure the opponent can't just take it.
2) Opening the position / giving my pieces more freedom
The two most common opening moves are e4 and d4. These moves release a bishop and allow it to move but this principle occurs many times in different guises throughout the game. If I can open a file which I can control with my rooks, if I can open a diagonal for one of my bishops, particularly if these are pointing towards the enemy king, then this is usually good.
3) Opening the position / giving my opponents pieces more freedom
If I have a pawn on a half open file or diagonal controlled by my opponent then I should try and avoid capturing with this pawn if at all possible. Conversely you may have a position where your opponent has a blocked pawn in the middle of the board which also blocks one of his bishops and so drastically reduces the effectiveness of that bishop. In that case it could be a big mistake for you to take that pawn with a piece even if the pawn is a free pawn.
4) Strengthening squares
I mentioned earlier that e4 and d4 are the most common opening moves. Apart from releasing a bishop these are good moves because they strengthen important squares in the center of the board. e4 strengthens d5 and f5, for instance. Later in the game you may get the chance to move a pawn to strengthen or take control of an important square. For instance your opponent (black) has castled kingside and pushed g6. If you get the chance to play g5 you will be exercising a lot of control over the weak squares f6 and h6.
5) Weakening squares
Say you (white) have castled kingside. Your white squared bishop has been exchanged. You want to move a pawn in front of your king to give it some air. g3 would often be a bad idea because of the way it weakens f3 and h3. h3 would usually be a better option.
6) To block or keep open
You have pawns on g6 and h6 opposite pawns on g7 and h7. Your opponent plays g6. Should you play hg or h6 or do something else and leave the position tense (unresolved)? It all depends on the what other pieces are on the board and where they are. It depends on the dynamism of the position.
If you have your rooks already doubled on the h file or can get them there very quickly then you are going to chop off hg very quickly and crash on through.
If there aren't many pieces left, queens and rooks are off, just the odd knight and or bishop along with the kings and a few pawns then you will probably favour h6 fixing the position. You then have the possibility of later trying to arrange to play Nxg6 or Bxg6 so that if he retakes with the h pawn then your h pawn is going to queen.
If you have your queen and bishop lined up on the a1-h8 diagonal then you will probably want to leave the tension unresolved so that if he takes gh then you can take the h7 pawn. You would try and build up more pressure with your other pieces.
What you should take away from this is a bunch of questions to ask yourself when considering some pawn structure move.
1) Does this get me closer to queening a pawn?
2) How does this affect the mobility and possibilities for my pieces?
3) How does this affect the mobility and possibilities for my opponent's pieces?
4) Does this strengthen some important squares?
5) Does this weaken some important squares?
Quite often only one of these questions will be really important but sometimes several of them can be important and trade-offs are required. For instance, this move reduces the mobility and attacking possibilities for my opponent but it also reduces my mobility and maybe weakens an important square while also strengthening another important square. Balancing all these things up is tricky.