I made an excess HSA contribution in a prior year and didn't withdraw it before the deadline, I am no longer able to withdraw the excess contribution. How do I correct?

If you are under HDHP coverage this year (or will be soon), then the easiest thing is to treat the carryover like a personal contribution in the current year, i.e., the year to which the excess has been carried over.

This means that you may need to reduce the amount you were planning to contribute this year, or else you just create a new excess.

If you can do this, then the carryover is "used up" in the current year, and there will be no more carryovers or 6% penalty.

If, however, you are no longer under HDHP coverage or have gone on Medicare so that you will never be able to contribute to an HSA in the future, you either live with the 6% penalty every year or you make a distribution, not for medical expenses, so that this distribution is taxes as Other Income and you are hit with a 20% penalty.

Which you choose depends on your situation - do you have any expectation of being able to contribute to an HSA again, and how much do you have in the HSA?

If you are eligible this year, reduce your current contributions.  You can "use up" the excess by applying it to this year's limit as long as your current contributions plus the excess is less or equal to your current year limit.

If you are no longer eligible, and plan not to be eligible in the future, but you will spend the account down to zero in less than 5 years, then leave it alone, the tax and penalty for removing it will be more than the penalty for leaving it in.

If you are no longer eligible, and plan not to be eligible in the future, but you will not spend the account to zero in 5 years or less, then you can withdraw the excess amount, and not use it for medical care.  It will be subject to income tax plus a 20% penalty in the year you remove it, but then there won't be an excess going forward.