Acer TravelMate P245-MG - Manual (Page 34)

34 - Changing to Windows 8.1 or Windows 7
3. Select Windows 8.1, and then click OK and follow the instructions.
Windows will restart, replace Windows 7 with Windows 8.1, and then
start the Windows 8.1 setup process.
Changing to Windows 7
Note
After configuring Windows 8.1, enter the BIOS (press <F2> when starting
your computer) enter the Boot menu and change the Boot Mode to UEFI.

See Changing BIOS settings on page 35 .
Note
To change back to Windows 7, you must change Boot Mode back to Legacy
BIOS. Windows 7 will not start if the BIOS Mode is set to UEFI.

See Changing BIOS settings on page 35