Your first step is to find an attorney in your area to advise you. Next, inform the landlord that you will be leaving the business and request to be removed from the personal guaranty. If the landlord says yes, you'll want the attorney to help draft the necessary amendments to the lease/guaranty.
If the landlord says no, you'll want the attorney to draft a document whereby the remaining partners agree to indemnify you if the business defaults on the lease and the landlord comes after you. Make sure you do this as part of a coordinated exit plan with your partners.