How can I download 14.8.1 for iPad 5th Generation?

The iOS and iPadOS update system have never allowed you to pick a release to install. They offer the latest release, and only the latest release, compatible with the detected hardware.

The only choice with the update system is whether you choose to install the latest update, or not.

Too late for that, now.

It is either upgrade to the latest iPadOS 15.1 version OR stay on the version of iPadOS your iPad is currently running.

Good Luck to You!