Your HSA provider should be able to give you an excess contribution distribution form, and help you determine how much to ask for. I had my contribution, my employer's contribution, and a very small amount of interest. I had stopped my HSA but not soon enough. They advised me to go 6 months back from the date I will start Medicare, and to include both my contribution and my employer's contribution in the amount to ask for. They would adjust as needed to remove the excess interest paid. The HSA account provider will send me the necessary forms to file with my tax return (to pay income tax as usual, since it is the pre-tax amount that I am getting back). However ... I have also found many indications that it should be 6 months before I apply for Medicare, not before I actually start, so I will back it up another couple of months to ensure I don't get a penalty.
