I want Mojave back

I want to restore my MacBook to Mojave, I have 7 applications and their associated files I would like to save, if possible. Notes is the only one that might be version specific. I have not used time machine, nor ever backed anything up. I have no idea where to even begin, because I am primarily a Windows/Linux user, this MacBook was a gift, that I do not know how to use. It ran Mojave when I first got it, I accidentally installed the new update last week and want to play my 32 bit games again, but the storage is too small to effectively run a VM. Is there any way to save these files, (too big for a usb and I'm too broke to buy an external drive, else I'd just use a VM) or do I just have to cut my losses and factory reset? (and how do I do that?) thanks so much, google has been infuriatingly unhelpful.