For $ 19.99 USD for 12 months of online multiplayer, free NES and SNES games and more, the Nintendo Switch Online service is a great deal. Whatever your reason, you can cancel the automatic renewal of the service at any time.
In a web browser
The automatic renewal of your subscription is easy, especially since you have paid all your time in advance. In a web browser, navigate to accounts.nintendo.com and log in with your e-mail address / username and password. Click on your account page in the left menu on "Store menu". This will open a new tab in your browser.
Scroll down to the "Shop Menu" page and click on "Nintendo Switch Online". On this page you can view various details about your subscription and the status of your automatic renewal, as well as a large red button on the right.
Click on "Disable automatic renewal" and then on confirm your decision by clicking on "Disable" in the small pop-up window. You will receive an email confirmation from Nintendo and you can still use Nintendo Switch Online services until the "Expiration Date" on this page.
On the Nintendo Switch
You can switch off automatic renewal on your Nintendo Switch device just as easily. Open the Nintendo eShop app on the home screen of your Nintendo Switch and select the correct user. Press the left joystick on the right to select your profile icon in the upper right corner of the screen; then press A to confirm. This will take you to the "Account Information" screen.
Scroll down to "Nintendo Switch Online" and then select "Disable automatic renewal". Confirm your choice and you will receive an email confirmation from Nintendo. You can still use Nintendo Switch Online services until the "Expiration Date" on this page.
You can register again at any time. But after 180 days, Nintendo will delete all Save Data Cloud backups on its servers.