How do I change the granny gear on a cheap Walmart mountain bike?
Depends on your budget, your requirements, and your mechanical aptitude.  Plus how far down the slippery slope of knock-on upgrades you want to do on a BSO bike.

Since the crank is riveted together, its unlikely to accept any bolt-on replacement chainrings.  Though its possible the makers used rivets instead of bolts but used normal bolt chainrings.  You'll want to pop the chainrings off and have a close look.  It may be possible to buy a MTB triple chainring like what you want, but it won't be walmart-grade or price.

Second problem is Front Deraileur capacity.  Most front mechanisms have a maximum tooth difference that they can swing, and for your existing FD is already coping with a difference of 18 teeth.  A difference of 22 teeth might be too much for it.  Hard to know.

Finances comes into this as well - remember a walmart bike is at the bottom of the quality scale.  I see little point in throwing good money at a crappy bike, when you can save up and get something better.  Don't discount used bikes either - a 5-10 year old used bike could cost as much as a new chainring triple crankset, but have a lot better quality components.

You say you want to ride through long grass - can you expand on why this needs such a crawler gear ratio?  I can ride (very slowly) up a 25% grade on a 26/24, and a 20/34 would be quite a low gearing.

You'll have better luck with the rear cassette upgrade.  I upgraded my 7 speed MTB from a maximum 28tooth to a 34 tooth megarange and it worked perfectly.  All you need is a long rear deraileur, and many bikes already have them because they fit the "look" even if the bike doesn't need them.  This will also decrease the tension on your chain - as the front chainring gets smaller chain tension increases.

TBH don't discount the value of an engine upgrade - ride your bike further for endurance or harder for increasing power.  This helps immensely, as does weight reduction of you and your load.