You should hire a criminal defense attorney as soon as possible. The sooner you act, the more options you have to reverse your plea.
You can call me at 612-223-7286 to discuss your case in more detail.
Whoever you retain, you have some big problems and an uphill fight ahead of you. I am not familiar with the law in Minnesota, but it would not surprise me at all to find that you have blown your deadline and that you may be very limited in what, if anything, you can do, at this late date. Furthermore, I do not know any state in which changing your mind is grounds for withdrawing a guilty plea. So even if you are within your time limits you may not have much to work with. But by all means consult a Minnesota criminal defense attorney and explore your options.
Mr. Sach is correct, it is incredibly difficult to withdraw your plea after you've been sentenced. But there are circumstances that make a plea withdrawal appropriate. You should contact a criminal defense lawyer to discuss whether this is a viable approach for you. Good luck.
I am a CA lawyer and I know very little of MN state law. However to directly answer your question, you need to consult with a criminal writs and appeals specialist. Many criminal defense attorneys specialize in trial work. Ask about their writ experience and their appellate experience. You need an appellate specialist to raise any challenge to the trial court's sentence (and after a guilty plea, your outlook is not good). You may need to seek out a writ, or habeas corpus specialist to challenge the plea. Find a lawyer who can do both. Good luck.