Go to account management and change your account status from Gold to Silver.
Edit for clarification:
1. On NXE, under My Xbox, click on your profile.
2. Go down and hit Manage Account.
3. Click on Memberships.
4. From here, you can modify your membership status to Silver or Gold.
Keep in mind that the subscription you're currently paying for will need to run out the rest of the month, and after that, you won't be billed for another month.