Note that it might be possible to do so while getting paid. I know of a guy who years ago applied for a job with Borland in SDK tech support. They gave him an open book test, saying "if you can find the answers in the manual, we'll hire you." After years of doing tech support, since he was immersed in the info day in and day out, he became one of the most knowledgeable C++ programmers I've met. Other potential paths include starting in QA, or tech writing.
Take classes. Khan Academy, Coursera, Udacity, and EdX offer free classes.
BUT it will take time. If you're not willing to spend the time, pick another field.