How to clean a garrison dehumitfer air filter ?

They do, because it is a refrigeration unit. Just make sure, fans and air filter screens are clean.

First remove the bucket. Easily clean the filter w/warm soapy water either under lightly running water or carefully gently clean w/towel. Tap lightly to remove any excess water. Place filter up to air dry which shouldn't take long. After putting filter back in, press. & hold the clean filter reminder button for at least 3 seconds.

A dehumidifier works in a similar way to an air conditioner, cooling the air that passes through it so water vapour will condense and collect into a container.

If the fans are working and air is circulating through the unit but no water is being collected the internal refrigeration system probably has a fault...

https://www.repairclinic.com/Shop-For-Parts/a14c9/Dehumidifier-Blower-Wheel-Fan-Blade-Parts

https://www.manualslib.com/products/Garrison-043-6121-4-8670428.html

Freon pressures are not correct, they are a little low. Also, check any air flow path and clean out any dust that may block the flow.

Wash it

http://marioc-domain.com/Download/manual_Download/Garrison%2070%20Pint%20Dehumidifier%20with%20Pump%20Manual/Garrison%20Dehumidifier%20043-6123-0.pdf ..this is all i could find on the internet

Here's a link to an article written by a service technician with more than 30 years of experience. It gives details on How to Clean Dehumidifier

the filter minder needs reset after you clean filter everytime Hold down for 5 seconds on button right next to light this should clear alarm

The filter in your Keystone KSTAD70B is meant to be washed. It is not listed as a replacement part in the owners manual. This is copied from the manual:

3. Clean the air filter
Remove the filter every two weeks based on normal
operating conditions.
Remove the filter from the top of the unit (See.Fig.11a),
or to remove the filter,remove Bucket then pull filter
downwards (See.Fig.11b).
Wash the filter with clean water then dry.
Re-install the filter,replace Bucket. CAUTION:
DO NOT operate the dehumidifier without a filter
because dirt and lint will clog it and reduce
performance.

If you have lost or destroyed the filter, go to Amazon and order the Duck Brand 1285234 Replacement Air Conditioner Foam Filter, 24-Inch by 15-Inch by 1/4-Inch, 1-Pack for 96 cents with free shipping as an add-on item to any order. It is also washable so you will be able to use it like the OEM filter material.