If you have any desire to leave the party, then first and foremost you have to write a special statement. Please note that it is made only to the Secretary of the nearest branch of the party (e.g., regional or local, if any). Not necessarily to indicate the personal reasons which prompted you to refuse party membership. Although, if you want to still be able to make them into a statement. Don't forget to sign the paper, put on it the date and personal signature.
Then take the application directly to the office of the party located at the place of your residence (if not local, then go to the regional office of "United Russia"). With a sure to take the party ticket, as you will have to surrender it when you exit the party.
Once your application is registered with the appropriate authority, you cease to be a member of the party. Thus the end of all your rights as an actor of the "United Russia", and also you will lose all the party's authority and cease to perform the duties previously imposed on you as a member of an elective Central and leading body of the party. It says in the Charter party.
Make sure that you submitted the letter of withdrawal was really signed and accepted by the Secretary. To do this, ask the appropriate entry with the document number and copy of admission papers. If it happened that your application is lost (that is, for some reason was missed), write it again and demand his personal presence during check in.
Carefully consider whether your desire to withdraw from the party is aware, or is it a momentary action. And it's not just that you in the subsequent years will not be able to join back. Consider the current political situation (such actions can greatly hurt your career).