You have a folder, you need to go to (depending on which version of windows you run)
7/Vista: Start -> My Computer -> C: drive (or whatever your primary drive is) -> Program Files (x86) -> Folder you installed Ultima PSO in -> Data -> drop in here.
XP: Start -> My Computer -> C: drive (or whatever your primary drive is) -> Program Files -> Folder you installed Ultima PSO in -> Data -> drop in here.
It's going to ask you to overwrite a file, before doing this, make a backup of the original file by right clicking on the file and selecting copy, then right click off the file and hit paste, then rename that to filename.extension.bak so for example if the file is PLtex.afs it'd be PLtex.afs.bak.
Hope this helped?