You'd be asking the judge who sentenced you to 75 years to resentence you to timeyou already served. Assuming that the time you have spent in jail and on parole would be allowed under the range of possible penalties for the charge(s) for which you were convicted, your motion would probably be unsuccessful.
For one thing the sentencing judge has heard all the evidence, and by giving you 75 years in the first place, he is telling you just how serious he thinks these charges are. If you bring a motion, you'd have to have some compelling reason for the judge to want to ignore the evidence he's heard and his own sentence to give you time served. Also, the DA would certainly file papers opposing your motion. The complainants, if any, would be notified for their input.
Do not think of doing this yourself without a lawyer, or at least consult with a local lawyer to see how viable your motion would be before investing a lot of time, money and energy in this process. Like all post-conviction remedies, what you're asking for is a very long, long shot. Your best chances of success would be with a lawyer, but even there, the overwhelming majority of these types of motions fail.
Another alternative, which would not, at least, require you to expend large sums of money on legal fees would be for you to apply to the governor for a commutation of your sentence. I'm attaching the application form.