Hi
To clarify: pages can not have categories. only posts. posts are displayed ON PAGES, so it can look like a page has a category if only one post per page, maybe. but the category is actually with the post.
posts are not "put under" any category. the only way a post can appear in a category is if someone ASSIGNS it to that category. with the exception of uncategorized. if a post is assigned to no category at all, it will be marked as uncategorized. The about page typically is a page and thus not in a category, not even uncategorized. If it appears as being in a category then a post must be displaying on that page.
When you edit a post below the edit window you will find a section category. It is there that a post is assigned or removed from a category. So look at your list of posts and find the one that's on the about page and edit it - there you can change its category.
make sense?