Very easy.
Step 1: Drill off the top of the old rivets
Step 2: Use a punch or something to pop the rivet into the mast.
Step 3: Remove old mast base.
Step 4: Position new mast base in mast and mark holes for drilling
Step 5: Drill holes in new mast base
Step 6: Check inside mast for a seal - if one isn't there then I'd get a piece of foam (like pool noodle) and slide it up inside, then cover generously with silicone.
Step 7: Squirt silicone around mast base and slide into mast.
Step 8: Squeeze rivets into holes.
Step 9: Enjoy!