Just make sure everything is turned off, unplug PC, hold power button for like 30 sec
Put on wooden table, one part of ur body touch case while slowly unhooking all the power cables, unscrew PSU take it out
Do all that in reverse order to put new one in
Except the power button and stuff, that's just to make sure power is all out of PC
Watch utube videos or ask someone to video discord/Skype to help