$ git remote -v

$ git remote add upstream https://github.com/foo/repo.git

$ git remote add upstream https://github.com/Foo/repos.git

$ git remote rm upstream
$ git remote add upstream https://github.com/Foo/repos.git

Older (backwards-compatible) syntax:
$ git remote rm upstream
Newer syntax for newer git versions: (* see below)
$ git remote remove upstream

Then do:
$ git remote add upstream https://github.com/Foo/repos.git

$ git remote set-url upstream https://github.com/Foo/repos.git

...
[remote "upstream"]
    fetch = +refs/heads/*:refs/remotes/upstream/*
    url = https://github.com/foo/repos.git
...

Log message

remote: prefer subcommand name 'remove' to 'rm'

All remote subcommands are spelled out words except 'rm'. 'rm', being a
popular UNIX command name, may mislead users that there are also 'ls' or
'mv'. Use 'remove' to fit with the rest of subcommands.

'rm' is still supported and used in the test suite. It's just not
widely advertised.