PSUs arent a challenge to replace. Undo all of the power connectors of the old one, undo the screws in the back, slide it forward a bit and it should come out the side of the case easily. Take the new one, put it in, slide it back, screw it in. Plug in the connectors to where you had unplugged other ones earlier. If you need a connector that is not permanently attached to the PSU, there will be a bag of modular cables, just plug in and use the ones that you need.