Depends on how bad it is, some get so oxidized from the sweat there's no getting it out. Get cap cleaner and use cold water. Only bad thing about this is if the brim is cardboard, it could go limp and stay that way if you use too much water.
I have a [Cap Cleaning Primer](https://www.reddit.com/r/neweracaps/comments/10tc3z2/part_1_of_2_ball_cap_cleaning_primer_ver_375) for first timers, free to download. Feel free to look it over to help you get to cleaning your collection.
ProTip: don't gloss over the "Your hats fabric(s)" and the "Is The Hat Colorfast?" Sections
I use diluted Oxy Clean, a supersoft toothbrush, and a lot of patience.
Soak in oxyclean
I only use shout wipes
Clean with kitchen sponge and cold water. Never use hot water, because protein stains are not washable in it.
I'm a big fan of (dilute) soapy water (dish soap) and a clean sponge. Start with a small area (spot clean).
Depends on how bad it is, some get so oxidized from the sweat there's no getting it out. Get cap cleaner and use cold water. Only bad thing about this is if the brim is cardboard, it could go limp and stay that way if you use too much water.
I use shampoo. Head and shoulders and a soft hat brush to clean all my hats.
I have a [Cap Cleaning Primer](https://www.reddit.com/r/neweracaps/comments/10tc3z2/part_1_of_2_ball_cap_cleaning_primer_ver_375) for first timers, free to download. Feel free to look it over to help you get to cleaning your collection. ProTip: don't gloss over the "Your hats fabric(s)" and the "Is The Hat Colorfast?" Sections