Depending on what you mean by 7.62. You can't take a standard AR15 lower (5.56) and put a 7.62x51 (308) upper on it.

If you want a 308 you need an AR10 lower.

With the standard 5.56 lower you can do a 7.62x39, 50 Beouwolf, 6.8SPC, and a load of other cartridges that are the same length or shorter than a 5.56 cartridge.

Go to ar15.com and you'll learn alot over there.