You can't. Once they're built as a reserve, they're always a reserve. You can however get their strength to go up in 2 ways. You can change your draft laws - the higher the draft law, the more people will be put in your reserve units (assuming you invest the IC to reinforce them).
You can also go to the politics screen and hit the big "mobilize" button, which will allow all your reserves to reinforce to full strength. This will however cost your consumer goods need to increase by a few IC if you aren't at war - it will also generate threat on the people around you.
I don't know why you'd want them to become standard units unless you intend to mobilize and demobilize several times in a game (since all your MP that was invested into reserve units is *lost* when you demobilize - but the costs are so low it typically won't matter and you can just stay mobilized from then on). Regular units have slightly more EXP too - but they also cost you consumer goods to maintain

which makes them prohibitively expensive - most people who are trying to "optimize" either upgrade their stock units (to get them off screen so to speak and negate their CGI draw, or delete them to avoid paying for them).