You can change the user with
Subversion 1.6 and earlier:
svn switch --relocate protocol://currentUser@server/path protocol://newUser@server/path
Subversion 1.7 and later:
svn relocate protocol://currentUser@server/path protocol://newUser@server/path
To find out what protocol://currentUser@server/path
is, run
svn info
in your working copy.