docker image tag server:latest myname/server:latest
or
docker image tag d583c3ac45fd myname/server:latest
Tags are just human-readable aliases for the full image name (d583c3ac45fd...).
So you can have as many of them associated with the same image as you like. If you don’t like the old name you can remove it after you’ve retagged it:
docker rmi server
That will just remove the alias/tag. Since d583c3ac45fd has other names, the actual image won’t be deleted.