Are you replacing with existing phones that have already been registered on the system or phones you bought?
If bought:
Plug in the replacement phones phones, allow them to go through their download cycles. Once done look in the Phones page of the GUI and select Add phone, hopefully your replacement phones' MAC addresses are listed in the drop down box. Select and follow the instructions to add extensions.
If existing:
Make sure the phone is plugged in, select it from the phone page using the MAC off the back of the phone, and add the appropriate extensions as needed.
Both are quite simple procedures, but I can understand your trepidation!
You can delete the old phones in the Phone page too. Select and choose delete.