When I boot my PC, I have to manually extend desktop to 2nd monitor  

no Signal on 1st desktop while boot. what should i do with this problem ?

how can windows do this automatically?

and can i change my pc monitor to desktop 1 ? because when i Identify display in Settings my pc monitor detected as desktop 2

switch the monitors in the customize your display menu. I've ran into this issue before and doing that usually fixes that problem. Then select the monitor you want to be your main display and select the option that says make this my main display.

Demonlord4lf said:

switch the monitors in the customize your display menu. I've ran into this issue before and doing that usually fixes that problem. Then select the monitor you want to be your main display and select the option that says make this my main display.

my pc monitor already main display except while booting because its no Signal

Is there ever any display on your PC monitor? If not, make sure all the cables are securely plugged in. Sometimes, just unplugging and re-plugging them fixes the issue.

Gunzorla said:

no Signal on 1st desktop while boot. what should i do with this problem ?

Short of moving cables around, nothing. At boot time it is the video card that determines which is the first display to be used, there is nothing for Windows can do as it hasn't even loaded at that point.