Make sure that you are signed in to your Roblox account.

Then click on the cog icon

in the top right corner and select "Settings."

Click on the "Billing" tab on the left side menu.

Select "Cancel Renewal."

You'll be prompted with the benefits

which you'll end up losing

if you cancel your membership.

Click on "Yes, Cancel."

Then select a reason for canceling your membership

or alternatively click on "Skip."

Now your membership will have been canceled.