can i change the consensus algorithm for a network that has forked from github? is there any link to understand how can make changes to consensus
Yes, if you have forked then you are free to change the consensus code.
The consensus part is mostly in the
consensus/ directory of the https://github.com/ethereum/go-ethereum repository.
I don't know of any guide to make modifications to geth source. Since the code is always changing it is likely that such guide will be obsolete almost immediately.