How To Take On Seller's Mortgage?

Hi all! An offer we made on a property was accepted and I am wondering how I can convince the seller to let me take on their mortgage (their rate is 4% compared to the current 6%).

Is this a complicated process? Is there anything I can do to encourage the seller that this is a great option for them? Has anyone had success with this?

Any and all advice would be greatly appreciated -- thank you in advance!