how can I restart redmine?

Restart your webserver. How to do that depends on your webserver you installed Redmine in, and your Linux distribution. For Apache on Debian, I'd do a:

# /etc/init.d/apache restart

touch /path/to/redmine/tmp/restart.txt

Redmine will restart at the next page request. This is often the only way if you're on an instance where you don't have permission to restart Apache (shared hosting, etc).

You just need to stop WebRick by doing a Ctrl+C, if running Mongrel you sould try

mongrel_rails stop