----Install----
Open 'update/update.rpf' in OpenIV and enable editing mode.
Delete 'handling.meta' from "/common/data/".
Add the new 'handling.meta'

----Optional Extras----
Merryweather Mesa:
I've also changed the Merryweather Mesa's handling to its own handling data (it's normally shared with the normal Mesa).
To use this change you have to edit line 12489 of vehicles.meta in common.rpf/data/levels/gta5/ to:
      <handlingId>MESA3</handlingId>