You need to unscrew the cupseal counter-clockwise from the gun. Once the cupseal is out then unscrew the cupseal as it is two distinct parts. When this is apart you will see an oring size 10 replace and reassemble.
If you're having trouble getting this done or cannot find the right oring give we a call @(877) 728-2243 via www.gatsplat.com
and I'll help walk you through it.