If you have a sweat stained hat that won't come clean here are two things that have worked for me in the past.

First, try soaking it over night in a diluted solution of an enzyme-based cleaner. After soaking it over night, wash it normally in the washing machine with your regular laundry. This has been successful at removing most of the sweat stains on my hats.

If you have a really stubborn hole, take the hat to a dry cleaner. A friend recommended this to me and I did it once a hat where I couldn't get the sweat stains out. I felt stupid taking it in and I got some funny looks. But it actually worked, the sweat stains that withstood soaking came out.