There are dozens of guides out there on how to change, ranging from video guides to step-by-step photo guides to simple text guides to the guide in the PS3 manual itself.
I'd suggest using one (or more) of those rather than relying on something written quickly here.
As for moving files, to do this, you will almost certainly want an external hard drive. Connect that to your PS3 through USB, use the Backup Utility, swap the internal hard drives, then use the Restore Utility.
Backups back up everything and can take ages if you have lots of stuff on your hard drive, so you may want to delete anything you can easily get back (for example game installs and videos that you also have stored on PC).