I am fairly new to PCP air guns but sprang for an AEA Challenger Pro. I like it a lot. But I am not very mechanical, fostering this question. After about 2000 rounds through it I have blown the O ring inside the fill port. I have a new ring ordered. I am told it is a quick fix BUT there are no videos I can find or photos of how to switch it out. I have removed the tube .... NOW WHAT. Please help this newbe with the very basics of how I replace the o ring. Thank you