Website: Log into your account and go to "User Settings," and scroll all the way down. There will be an option of "Deactivate Account" click on it, and you'll be taken to another page. Put in your login details here and click on "Deactivate."
Mobile: Go to "Settings," scroll down and select "Help Center." This will take you to the Reddit help page. Type in the word "Deactivate" and search. You will come across the question regarding deleting the account and just follow the instructions to deactivate your account permanently.