Gearing change on Mongoose Malus
I can't find a clearly written spec page for this bike, but based on the picture your easiest change will be to swap out the cassette/freewheel for a 7 speed with a larger number of teeth. 

The question is whether the rear derailleur can fit a larger gear. 

You'll also need a new chain to add more links.

Your other option is to replace the whole front chainring assembly with a smaller one.   At this price point there's a good chance its a common square taper fitment, so you might even be able to rescue a smaller crank off a donor-bike.

You could even use a triple crank off a MTB but only hook the chain onto the middle or little ring.   This would be the most economical option, the only bike-specific tool you need is a crank puller.