How can I have a DD293 mailed to me?
I don't know who would mail that to you. These days forms are often printed directly from a site on the Internet to be completed and delivered to their destination. Some web sites present forms that are "fillable" so that you can fill in the data before you print it.
If you don't have a computer and printer setup, I'd suggest you head to a public library near to you and print the form for your use.
You'll want to chat with the folks at The Veterans Consortium for help with a discharge upgrade. These can be tough to DIY so getting some expert help is a good idea.