@user151569, welcome to Meta Stack Overflow! Basically, for every post, you can choose exactly one of the following three options:
- vote up
- vote down
- not vote at all
Each option has slightly different reputation requirements. In general, you cannot undo or change your vote. There are two exceptions:
- immediately after you vote, you can undo/change; this is in case you clicked the vote button by accident
- if the post is edited, you can undo/change; this is in case the edit changed something that you previously agreed/disagreed with
For more detail, see the community-generated FAQ here. The community-generated FAQ is not the same as the site FAQ that you were shown earlier.
You can vote for each question and each answer completely independently. You cannot have multiple votes on one post at the same time.