Some more info on top of what @MeNoTalk said:
It mostly depends how full the airplane is and why you want to switch. If the airplane is rather empty, there is no issue, you do not need a reason and you can switch as you like. If it is full, it depends how personal the reason is and if another (higher or lower) class is still free or if it is acceptable to get someone else into the seat.
- If your seat is broken, they will not put someone else there, same if it is next to a baby etc.
- If you are a woman and can argue that you feel uncomfortable sitting next to a specific man, they will most likely re-seat you.
- If it is about sitting next to your friend/spouse, you can simply go and find someone single to switch with you or your friend. Make sure that the other person switches to the better seat (towards the front etc)
- If the staff is very busy, you likely better help yourself and find someone who would switch with you.
- If you see the issue right away, ask immediately during the boarding procedure if the airplane is full. If it is not, either sit down or stand at an emergency exit and wait for the doors to close. Make sure you get your hand luggage into a bin already however.
- Discretion and politeness is 99% for a good switch. Talk to the staff in private (as mentioned) and indicate that you are fine to switch later, too. So you take the pressure off them. Try to indicate your wish to switch as your problem/weakness instead of a demand towards the staff.