Available options are double side-band mixer (simply a signed multiplier) or single sideband mixer (quadrature modulator). A digital imlementation should be more or less obvious, I think. Otherwise, there's a lot of literature about SDR on the internet.
There we go. Anything more falls outside the detail level of your specs. Problem solved.
And I wasn't going to be the first to point it out, but since FvM already did, I'll be the 2nd to point it out. Google? When I was looking for gps sdr implementations (which is a narrower field than random generic anything SDR as per your question) I had no problem finding boatloads of information. So google is your friend.