I would like to standardize the usage of tags and re-tag some questions, but I don't see the "retag" option under the questions, I can only flag it or edit. Does it mean I should just edit the question even if I am only interested in changing its tags?
That's because you have more than 1000 reputation now, which is the limit you need to be able to edit posts without review. Just use the edit link now, it does the same thing as the retag link you used previously.
The only difference for retag is that it needs less reputation.
If you mouse-over the area immediately to the right of the existing tag list, a link appears that allows you to edit tags on the question page rather than going through the edit page, might save all of ten seconds =D