You have a couple of options. The first one is that you can go to your main Runes of Magic game folder (C:\Program Files\Runes of Magic, or C:\Program Files(x86)\Runes of Magic) and edit the Client.exe.ini file. It should look like this for version 5.0.1.2553:
[Version]
Major=5
Minor=0
BuildNum=1
AutoIncrement=1
Extend=2553
Country=en
If editing the file doesn't work for you--it should, but odd things have been known to happen--you will have to download the current Chapter V installation files for the client you wish to use, full or slim, and reinstall the game.
Hope this helps.