Steps to change a blog's URL (web address) in Blogger:
1. log into Dashboard
2. click on the relevant blog
3. click SETTINGS tab
4. click PUBLISHING
5. edit url (see screenshot above. Place where you type the new URL is indicated by cursor (arrow) and also "Type new URL here" in red
Remember to save settings.
Your success will depend on whether the URL has already been taken up or not.
NOTE: Remember, if you changed the URL of your blog, any existing incoming links will become dead link. If your blog is relatively new, that may not be a problem, but if your blog has been on the Web for a long time, your blog will already have been indexed by search engines, and you will have to wait to be indexed by the search engines all over again, and for some time, people searching for content in your blog will get links that lead nowhere. You will lose your PageRank and will have to wait a long time to get a meaningful PageRank for blog with the new URL. People may have linked to your blog or posts and all these will become dead links. Further, there have been lots of reports of bloggers who deleted their blog or changed its URL. Sploggers grab the now available URL and use them for dubious uses the worst of which is pornography. This will be extremely embarrassing for you if your family, relatives, friends, colleagues, etc., have your old URL.
So if yours is an old blog, think very carefully before you change the URL. If you insist on doing it, I would suggest immediately try to re-register your old URL and use it to redirect the old blog to the new blog URL. See Redirect New Blogger blog to another site