Cancelling your Insight Notes backup and encrypted exporting subscription occurrs via your Apple ID account preferences, which is where you manage all of the subscriptions you have purchased through Apple.
Apple has a handy guide featuring step-by-step instructions: https://support.apple.com/en-us/HT202039
Before you cancel, be sure to export your notes to yourself so that you have your own copies. If you ever decide to return to Insight Notes, simply navigate back to your subscription preferences (see above) and turn the slider back to "ON."
Finally, remember to write down (1) your encryption code, (2) the email address you used when you signed up, and (3) the password you designated when you signed up with the backup service. Keep them in a safe place. Those credentials can't be reset and we don't have access to your notes to send the to you, nor can we mine/hack our own server to retrieve them for you.