Newsflash: Best way to take over parents house?

I want to take over my parents mortgage payments but want to put the house and loan under my responsibility/name. Their current loan is assumable, but they have a variable rate and monthly payments doesn’t even cover monthly interest. House is currently worth $940k but the mortgage is $550k still.What is the best way to go about this to not pay too many fees? Do I assume the loan, and refinance in the future ?Do I buy it from them ? And if I do buy from them how would I even do that? Because they won’t ask me for more than what mortgage is