Unlike Crosman, Gamo doesn't offer drawings on how to take there rifles apart. It would be hard for anyone to tell you how to take your apart, because you didn't say which model you want to replace the spring in. So I suggest one of two choices. Go online and do a search for "airgun repair" and locate someone who repairs airguns near you and take it to them, or contact Gamo customer service directly and see if they will send you a parts drawing of your model. ( See the link below.)
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