Airguns are surprisingly hard to work on...
That level of airgun isn't really meant to be serviced. When they wear out, just buy a new one. Or, if you want to go to the trouble, there's a good chance that the manufacturer will service or replace (more likely) it for you for free or at a discount if you send it to them.
If you get into the nicer models, there are airgunsmiths who can repair and tune them.