Creating and editing components is a skill an EAGLE user should learn early on.
Refer to your owners manual for detailed information.
Here is the Readers Digest condensed version.
Make sure no schematic or board files are open.
Open Heatsink.lbr for editing by right click > open.
In the Package tab (looks like an IC) open SK129PAD for editing.
Remove the polygon in layer tRestrict.
Open your board file and and do Library > update all.
Your heat sink should no longer restrict your ground plane.