Help to Update HTC One M8

I bought a used M8. It is unlocked. It currently has a very outdated version of software. Software number is 4.18....
When I checked for software updates, it says the phone is up-to-date. Please help me to get my phone updated.

I searched the IMEI info for this phone. Part description says it is a USA device, AT&T, with SIM lock.

Therefore, my assumption is that the phone was AT&T locked but was subsequently unlocked.
Could you please advice me on whether I should flash unlocked ROM or is...

Help to Update HTC One M8

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