You will need to get to Settings to turn off tablet mode.
To get to the Settings, tap or click on the bars in the upper left hand corner of the desktop screen.
Now tap on or click on System.
Now tap or click on Tablet Mode.
From here you can select Use Desktop Mode from the drop down menu.
NOTE: Also, ensure that you choose the Don't ask me and don't switch option from the When this device automatically switches tablet mode on or off drop down menu.
Turning Tablet Mode On and using the Action Center Message Button while it is in Tablet Mode
Tap or click on the Action Center’s Message Button icon from the lower right hand side of the taskbar.
Now, tap on the Tablet Mode icon from the action center to get to the Settings and follow the previous instructions for the settings.
Or you can Swipe left from the right edge of your touch enabled PC to open the Action Center.
Then Tap the Tablet Mode toggle icon. There’s a message button for the Action Center on the right side of the Taskbar too.