I fly using an iPhone, so it's slightly more complicated for me, since there is no way of fooling iOS about your location. I have to use an Android device to make the change, and then remember NOT to change it back when using my iPhone.
Therefore I have written the instructions out for myself.
(If you don't use iPhone, you only need the first two sections below).
Reset Mavic Air to FCC Mode Using Android and Android App
Preparation
On the Android, download and launch the app Fake GPS
Remove gimbal protector from Mavic Air
Power on the Lenovo
Android
Connect Mavic Air controller to Android
Launch the app Fake GPS
Locate Texas and choose play button
Start the controller
Start the Mavic Air – no need to start motors
Wait until they are linked (red light on controller goes green)
DJI Go 4 will automatically start on the Android; if not, launch it
Wait until Enter Device > has gone and press Go Fly on DJI Go 4
Ignore any requests for a database update
DJI Go 4 will report that the Wi-fi has to be reset:
Switch Wi-Fi Settings
Your aircraft is flying in a different country/region. Aircraft will reset its Wi-fi...
Click OK.
Unplug the Mavic Air Controller from the Lenovo, and turn it off, then turn the Mavic Air off
iPhone
Connect the iPhone into its usual position in the Controller, and turn the Controller on, and then the Mavic Air
When the Controller and Mavic Air have finished their startup and binding (red light on controller goes green), launch DJI Go 4 on the iPhone
Cancel and ignore any update warnings
DJI Go 4 will report that the Wi-fi has to be reset:
Switch Wi-Fi Settings
Your aircraft is flying in a different country/region. Aircraft will reset its Wi-fi...
This time choose Cancel.
Check the wi-if symbol has a small 2.4G beside it.
The procedure is also in the .pdf attached.