My new Internet box appears twice .....

Search for Device Manager and open it. Expand network connections (or similar, I am not at my computer at the moment). Select your old adapter and disable it. With the old adapter disabled all wireless connections will use the new adapter. I'm not sure about Windows 11 but in older Windows you had to type the password for the Wi-Fi once again because each adapter had a separate known networks list.

What are the download and upload speeds on your ISP plan?

If it's 150 Mbps, a Wi-Fi 6 card is pointless, and the motherboard's built-in card is perfectly adequate for those speeds.
That has already been mentioned.

If you leave both wireless adapters enabled, you should see a selection top right of the list of available networks to switch from one adapter to the other. The old should be number 1 and the other number 2.

Hello all !
I received my WiFi 6 adapter and installed the appropriate driver
Thhings are much bet ter !
I'm able to "switch" between the 2 adapters and everything seems correct and coherent when I look at the WiFi parameters in Windows 11
One poblem remains : the speed
My wife's computer (in the same room where the Livebox i installed) fully benefits from the optical fiber and runs "full speed"
My computer (one floor above the LiveBox and only a wood floor separating them) does not show any improvement compared to the previous ADSL connection
So I ran some speed test with a phone - Close to the Livebox it gives 31,6 when the same phone close to my computer drops down to 4,84

So, I suspect it's a loss of signal power and I'll probably buy a WiFo Repeter

I still have one concern about the Livebox
If you look at the attached file, it seems the protocole used is WiFi 5 (802.11 ac) when it should be WiFi 6 (802.11 ax)
Could I change it ? How
(I attach the capture of the adapter settings)
Anyway, thnks every one for your time and the help you provided

Have a good week !

PS : Please excuse my poor wording ... english is not my native language

If you set the maximum protocol (last on the list), then Windows chooses automatically the highest protocol the router gives. I am not sure but the exception could be if the signal is too weak for 802.11ax, it then chooses the lower one 802.11ac as you see in your screenshot. If your house is a two level house sharing the same electricity cables, you could buy a wireless powerline pair. One device connects downstairs taking the network directly from the router and sharing it through the electricity cables. The other device connects upstairs taking the network from the electricity cables and then giving it wirelessly, much like when you make your phone a hotspot. This should give you maximum speed. Just read the specs carefully before buying, make sure the LAN supported by the powerline is Gigabit Ethernet, or speed will be limited to 100Mbps! Also make sure the wireless network is at least 802.11ac, not 802.11n or lower. If your house is two separate levels and don't share the same electricity cables, then you have to buy a wireless repeater (also known as wireless extender) and make sure you place it upstairs at the point where the signal is strongest so it spreads it around the whole level. There are apps for your phone to test the signal strength. However the powerline will give better speed.