How do I change the granny gear on a cheap Walmart mountain bike? I have a cheap ($100) 26" Walmart mountain bike (Hyper Summit) that has a triple chainring (24,34,42).  It appears there is no easy way to remove the granny gear since it has what looks like some type of rivets.  However, the BCD (Bolt Circle Diameter) appears to be 58mm which is a standard size.  I would like to go with a 20 granny chainring cuz I like to ride thru tall grass and need a lower gear.  My rear gears are a 7 speed freewheel with a 28 as the largest cog so currently, 24/28 is my lowest gear which is about 22 gear inches (26" tires).  I will eventually be changing to a 34 largest rear cog (Shimano MegaRange 7 speed freewheel) but I also want the change the front granny chainring for a double gear reduction which I know I will like.  So any ideas how to get the granny gear off?  It connects in 5 places (starfish pattern).  My goal for my lowest gear is 20/34 which will give me about 15 gear inches.

I was thinking if I can somehow get those rivets out without damaging anything critical, I can probably use some washers and chainring bolts to handle any difference in hole sizes.  It seems like it can work with just some light machining.  Once the chainring is off I can have a bike mechanic work on it.  There is always a brute force method too.  For example, cutting the teeth off of the 24 chainring and attaching the 20 chainring to it.  However I prefer trying to bust out the rivets first as the BCD of 58mm matches up with available 20T chainrings.

UPDATE: I am moving forward on this project so will update when I get some actual results.  It might take a month or so to get the parts and schedule a bike shop appointment.  I think at minimum I should be able to get the different freewheel gearset installed but not sure about the 4 tooth smaller granny gear.  If I can get both it should be awesome cuz that will then be a low gear with 15 gear inches which is very low.
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.