There are some good articles on how to try to tame an aggressive rooster on here, but I think with this problem prevention is so much easier than cure. With the right treatment and training from chick-hood, cockerels don't have to turn into mean roos, but once a roo has gone that way it can be very difficult to rectify unfortunately.
You should remember that your rooster doesn't hate you and that he is not being bad by being aggressive. He is simply confused because he sees things completely differently to how we humans see things. He has decided you and all people are a threat to his flock and it is very important to a rooster to challenge all threats. He pretty much has only two life purposes: to guard the flock and to propagate the flock through mating.
Changing his behaviour requires changing his mind about you being a threat. Obviously this isn't simple. It takes time and effort and can be dangerous and you might decide it isn't worth it for you.
If you want to try to retrain him, You have to show him that you are non aggressive but also not afraid of him. Never fight back and never run away. Trim and blunt his spurs if these are a danger. Take something outside with you as a shield and wear thick clothes or boots if necessary. Stand your ground and do whatever you need to in the coop or yard, ignoring him as much as possible. If he gets in your way, move through him, or pick him up and hold him tightly, or pin him down by his back for a few seconds. If he sneaks up behind you, turn and stamp the ground and stride toward him until he backs down. Never kick or hit him with anything, it will only prove to him you are something he must fight. When he isn't being aggressive, give him treats and speak to him to show him you can be a positive addition to his area.
If you can persist in these behaviours for a few weeks things may well improve. However there is a chance he will never stop his aggression toward humans, in which case the options are giving him away, culling, or finding ways to live with him with minimal contact between him and humans.
