Why is my software keeps telling me it’s up to date, and it’s an old version though 12.5

Because that's as far as an iPhone 6 can go.

Read this link for the easiest way to find out what your device can run: "Update your iPhone, iPad, or iPod touch" --> https://support.apple.com/HT204204

- When you try to update you will only be given one update selection which will be the highest available iOS version supported by your device. If it says the device is up to date and you need a higher OS version you will have to buy newer equipment.

The iPhone 6 does not have the required hardware to update beyond iOS 12.5.1.

Because it is almost up to date. The last one you can get for your device is 12.5.1.