To edit your favorite or ignored tags, you browse to the site home page, go to the list on the right side of the page, and click Edit. This causes a text box to appear that lets you enter a new favorite tag.
Clicking the Edit button should automatically focus the text box, since typing a tag is the most likely action.
Click This:
Nothing Focused:
See also Move the Add Favorite / Ignored Tags boxes to the top, a similarly helpful UI tweak.