How do you raise rpm on John Deere la145?

((I assume you mean beyond the highest throttle setting (using the throttle leaver).))

The lever should be attached to the carberetor by a cable. It may be possible to replace this cable with a longer one, but if so, it will allow you to run the engine a lot harder than it was designed for. You can wreck your engine this way (potentially in a spectactular and hopefully not life threatening way) in addition to voiding any warranty you may have.

If you do wreck your engine, most likely you will hear a loud *clank* and the engine will stop instantly. The engine will need *major* work done to it to bring it back to life, assuming it's worth the expense. I really don't recommend 'overclocking' your engine.