Is it possible to change a branch tag name?

[John R. Dunning]

One of the folks in my group has decided he wants to change the name
of a branch tag, after having done some amount of changing of files on
that branch.  I'd never heard of a way to do that, and a scan of the
docs doesn't reveal anything that looks like a facility for doing
that, but I figured it was worth asking the list:  Is there a way to
change the name of an existing branch tag?

Thanks in advance...