Finally

@TP-Link_Deco 

How about updating via Deco app?

@TP-Link_Deco  that looks very promising, but i don't see it in the download page in Norway yet, I guess I need to wait more?

With 3 Deco M9+ the update must be done individually 3 times with cable to each unit?

@cacb 

I recommend you doing the update connected to the main deco by cable

@SrDarkWarrior Thank you for the advice! The two ports on the main Deco are busy now, I have cabled backhaul between 2 Decos. Should I remove that cable to do the opgrade or the one going to my ISP fiber box? In other words, is it necessary to have internet connection during firmware update?

@TP-Link_Deco 

Am I missing something, if you can't access the web interface on the V2 how can you follow the instructions to update the firmware manually via the web interface?

@nictry There is a windows utility to do it https://www.tp-link.com/en/support/download/deco-m9-plus/#Utility

I do think TP-Link should push the downloads to the various regional sites, it would be less confusing. Enabling the upgrade via the current Deco app would be even easier.

Still nothing in Italy, while I know the new firmware is available is some Countries like NL, FR, CZ.

The local web tp-link sites recommend to dowload only  from them and for the specific Country, but I know people applying the available one without any issue.

That's a bit confusing.

I will wait some more time.

@Sleater14 

I think the update is the same for alle countries.

the update file had eu jp us in it's name if I remember it correctly.