The additional information is certainly helpful. With appropriate context, it’s much easier to provide appropriate guidance.
At present, you essentially have two choices. You can continue remain with iPadOS 14.8 - and continue to receive security updates - or you can update to iPadOS 15.1. Once updated to iPadOS 15.x, there is no way back.
If you don’t wish to update to iPadOS15, you might be well advised to disable automatic download of updates:
Settings > General > Software Update > Automatic Updates >
- Download iPadOS Updates - set to OFF
- Install iPadOS Updates - set to OFF
As the iPadOS 15.1 update has already been downloaded - but not installed - the update package can be deleted from your iPad before it is irreversibly installed:
Settings > General > iPad Storage
Scroll down the list of Apps until you find the update package; swipe to the left, then tap Delete.
Having deleted the iPadOS 15.1 update from your iPad, return to the update page - and install iPadOS 14.8.1 (at the bottom of the page). This update includes an essential security update.