Yes, you can change your vote as long as the voting phase is not turned off by the administrator. If the voting is turned off, all votes are "frozen" and you cannot change them. Until then all votes can be always changed.
You can pick between 2 voting options in your account settings:
Voting with votes: Here a user has a maximum 25 votes and can only vote up to 5 votes per idea.
Thumbs up or like button: Here users can give one vote (like) for an idea only, but each user can give a like potentially to each idea.
You can switch the options under "My Account > Account settings".
Be aware you cannot switch between these 2 options once you have started with one. Then you need to erase first all votes to switch to the other options.
Every user has maximum 5 votes per idea and 25 votes in total. So you can decide if you give for example 1 vote each for 25 ideas or 5 votes to 5 ideas etc.
If you see a thumb up then voting is reduced to only one vote per idea.