as you know bitcoin uses (0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798,0x483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8) as a generating point, my question is how to change the generator in secp526k1 (or append it inside pycoin package).

thanks in advance for your help

The generator point G is a part of the definition of secp256k1. If you change the G, then you are not using secp256k1 any longer.

And why would you want to change the G that is used. The choice of G does NOT affect the security of the curve in any way, since IF there exists even one weak generator for curve secp256k1, then it can be used in cracking the curve with any choice of G.

But to your question. You can create a curve with any curve parameters you want, but then you will have to give the curve parameters manually, since you are no longer using secp256k1...