To use alternate credentials for a single operation, use the --username and --password switches for svn.
To clear previously-saved credentials, delete ~/.subversion/auth. You'll be prompted for credentials the next time they're needed.
These settings are saved in the user's home directory, so if you're using a shared account on "this laptop", be careful - if you allow the client to save your credentials, someone can impersonate you. The first option I provided is the better way to go in this case. At least until you stop using shared accounts on computers, which you shouldn't be doing.
To change credentials you need to do:
rm -rf ~/.subversion/auth svn up
( it'll ask you for new username & password )