an additional note:
if you have already added a remote ($git remote add origin ... ) and need to change that particular remote then do a remote remove first ($ git remote rm origin), before re-adding the new and improved repo URL (where "origin" was the name for the remote repo).
so to use the original example :
$ git remote add origin https://github.com/WEMP/project-slideshow.git
$ git remote rm origin
$ git remote add origin https://[email protected]/WEMP/project-slideshow.git