How do I update Reminders if I did not do so when the iOS was updated?

Perhaps to review the accompanying article above "upgrading the Reminders app in iOS 13 or Later"

may reveal an issue which may keep you from upgrading yours. ~Should that device & others you've

there, not all be upgraded to the most recent.. (That could be a reason for those to not upgrade now.)

As a fellow user and occasional contributor here, those kinds of details could be of significance now.

PS:

Note: 'All Reminders' features described in this guide, are available when you use

Upgraded Reminders. Some features aren’t available when using other accounts.

re: Set up Reminders accounts on iPhone - Apple Support

Bring up the reminder and modify it. See especially items 2 through 4 under “Add a new reminder” here.

Add reminders on iPhone - Apple Support

Reminders are updated just the same now as they have been since iOS 13.