Assuming this is a PIN for Roku and not something required by Showtime ...
Your Roku PIN is entered or updated on your Roku account at https://my.roku.com. You can also set when the PIN will be required.
Be sure you are logging into the Roku account indicated by the email address shown via Roku menu path Settings > System > About.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
As previously mentioned by @makaiguy above, we recommend checking first the email address linked to your Roku device. To do this, go to Settings > System > About. Once done, log in to the same email address on our website then try updating your PIN preference.