How can I clean the swampy smell coming from my air conditioner?

What type of air conditioner is it?

This link should help, Jacki:

https://www.doityourself.com/stry/removing-bad-odors-from-an-air-conditioner

Check out any filters you have and give them a rinse in hot water and baking soda, along with a dash of vinegar.

Spray Lysol in the inside and outside

Jacki this article has a good explanation and solution https://www.cooltoday.com/blog/musty-smell-coming-from-central-a-c-vents-heres-why

Hi Jacki. I'm Cindy. You could put a dish of charcoal in the unit to remove smell. But, honestly, the smell will come right back. It sounds like mildew has been built up over time. I'm sorry but that's bad news. You need a HVAC man to come out and do a maintenance check. There are probably parts that need to be cleaned or replaced. Good luck Jacki.