They have docs for everything, here's the part specifically for programming, as well as what GPIO's go to buttons, relay, etc:

http://doc.mokotechnology.com/index.php?s=/1&page_id=149

You won't program with USB directly, you'll need a TTL programmer - this is the one I use, but there's lots of others that work just fine - just make sure to use on that is 3v:

https://www.amazon.com/gp/product/B07TXVRQ7V?ie=UTF8&psc=1&linkCode=ll1&tag=ana024-20&linkId=33729a9920fd20c235821e792efe3f61&language=en\_US&ref\_=as\_li\_ss\_tl

This (https://www.amazon.com/gp/product/B07TXVRQ7V/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1) with a USB extension cable works perfectly.