Why is my iPhone 6 Plus not updating to iOS 12.5.4

Go to---> Settings---> General---> Software-update---> Download and install the update.

You must have a stable Wi-Fi connection.

Or connect to a PC with the latest version of iTunes installed.

Charge the battery to 50% or higher.

Turn off Battery saving mode.

Don't use a Hotspot.

Don't use VPN.

And make sure you have enough free space.