Select the textbox and type in the new shortcut. Use the keyboard to type in the new required shortcut to be used
Shortcut already assigned
If you are trying to change a shortcut of an action that is already assigned to another function - it will give you an error that says the shortcut is in use. You will need to find the action that is using the shortcut and assign it a new hotkey and then you will be able to use it for your assigned function.