How do I get a safe update for an ET51 stuck on Android 8?

Start with Zebra support or the reseller that supplied the tablet. A customer account may be enough to access the downloads, and support can help identify the correct package. Have the exact ET51 model, current build number, and region or configuration ready, since a package for a different variant may not be suitable.

Be careful about jumping straight from Android 8 to an Android 14 full update. A recovery signature or footer error is a reason to stop, not to try different flashing tricks. The update may require an approved sequence of intermediate packages, or it may not match that tablet's configuration.

I would avoid relying on a temporary file-sharing link for firmware. Even if the filename looks right, you still need to verify that the package is genuine, intended for your exact ET51 variant, and compatible with its current build. Follow Zebra's instructions for the package rather than assuming a standard recovery or ADB method will work.