Ryan, handle?? Do you mean the pistol grip. If so there should be a screw you can tighten in the bottom of the grip. Not sure don't own this model but there might be a base plate at the bottom of the grip you would have to remove to get to the screw. Should just pop off if there is one. The quickest route would be to take it to a gunsmith. You can also contact the manufacturer at (706) 701-5554 or you can email them at (cs@heckler-koch-us.com.) they should be able to walk you threw installing it or start a warranty claim if necessary. Sorry you're having trouble but hope this helps some.

Ryan, you could send it back to H&K for warranty work, but it may be easier/faster to take it to a local gunsmith.
H&K Customer Service (706) 701-5554. All new HK firearms sold in the USA are covered by a Limited Lifetime Warranty for the original owner.
Stay safe & have a great day!

Hi Ryan and welcome to the community. Please contact H&K's Customer Service at (706) 701-5554 for a resolution. All new HK firearms sold in the USA are covered by a Limited Lifetime Warranty for the original owner. Cheers and stay safe!