How can I access another computers registry?

Remotely accessing a registry is almost the same as any other remote
administrative task. Open regedit, then file->connect network registry… key.
Like other networking tools, you need rights on the other machine. If you
are on a domain, then your domain account needs to be mapped to an
administrative group, or if not on a domain, you need to have the same
username\password as an account on the target. Otherwise you’re asked for
a username\password with the permissions.
For massive registry changes over groups of computers, group policy is
recommended.