To change the branch you have checked out you use “svn switch url [PATH]” (the path is optional).
example:

svn switch https://example.com/svn/root/branches/thebranch .

Also svn switch can be used to “relocate” the repository url – i.e. if the server has moved/changed address/ip or similar. “svn switch –relocate FROM TO [PATH]”
example:

svn switch --relocate https://old.example.com/svn/root/oldbranch https://new.example.com/svn/root/newbranch .

warning: Only if the working copy still reflects the same repository directory, but the location of the repository itself has changed, then svn switch –relocate is safe to use.