You can take a lot on the below threads:

[1] 51% Attack
[2] 51% attack on Bitcoin
[3] How exactly would a 51% attack work?
[4] Bitmain 51% Attack? Would they do such if they get the hash? [POLL]

Please take note though that some of the threads are as old as 2011, meaning this topic has been discussed many times in the community.