Change to the directory which contains the client's (or users) main directory, then issue:
chown ownername:groupname -R directoryname
If you want to leave the groupname unchanged, then do not specify one, just put the ownername: (with the colon).
Be careful if changing the domain's root/home directory, the owner/user name should be the same as the ftp user name for that domain, and the group should be psacln.
The -R tells the command to do it recursively. You can also put -v to have chown list all files/folders which are changed.
Before doing this, please tell us what directory structure you are thinking of changing the ownership on, so we can verify that you will not cause additional problems on your server!