Software version 12.4.9 does not update

Your iPad Air1, with iOS 12.4.9, is capable of update to iOS 12.5.1 - this being the most recent release of iOS for your iPad.

These support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties with the update process:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

It should update to 12.5.1.