Comment Depends on what you mean by STEM (Score 3, Interesting) 280
Unfortunately STEM is quite broad. I'm an Electrical Engineer by degree who has worked in software development and testing his whole career. In that time I've seen people from various backgrounds get into dev work. Fresh out of college I worked with a systems integrator who specialized in AMX and Crestron development who had a liberal arts degree. I know a few MCSE's who were high school teachers who now work with Microsoft Sharepoint. None of these people went back to school for more formal (expensive) education. Most either fell into it or did this sort of thing on the side as a hobby, and the hobby took over. My suggestion is look at what you can already do and start pursuing that. If you repair computers for friends and family start looking for entry level tech work. If you write code and can produce some sample applications (or even better publish some useful apps for IOS or Android) then use that as a stepping stone to apply for code work.
I do not recommend you head back to school or join the military for this purpose. There are better resources at hand. If you're looking to code codecademy.com is an excellent resource. If you're looking for more STEM education coursera and kahn academy are excellent resources to brush up with. I would recommend certs to prove your worth. Find jobs you are interested in and see if they require certain certifications. Get certifications that are affordable and in your wheelhouse, but don't go after a $2000 certification that only a few places want you to have. In most cases your skill should get you in the door, and a company should be willing to get you certified at their expense to sell your services.