Yeah, as austin said, youtube's a great place for finding help, but rarely in the comments section. First off, you'll want to ground yourself by touching a metal part of the case before touching anything inside of the computer, static electricity can damage the components.
I'd suggest taking a look at where all of the cables from the PSU are connected first, and memorize that or write it down. Make sure your new PSU has the same connections, plus anything else you may be adding. Unplug all of the old PSU's cables, remove the four screws on the back of the case holding the PSU in place, and slide it out gently. Try to avoid bumping it against any other components. Slide the new PSU in the same spot, secure the screws, and plug everything back in the same way. If you have extra cables, use plastic cable ties to hold them out of the way. Make sure that none of the cables are touching anything other than the case, especially the fans. If the new PSU does not slide in easily, don't force it, you don't want to break anything. Measure the old PSU, and make sure the dimensions are the same.