Hover your mouse cursor over the entity you would like to Unlock until a Yellow Info box appears (wait 1 second), then press 'U' on your keyboard, the layer will then be unlocked.

NOTE: If it seems like this function is not working, press ESC (escape key) on your keyboard and then try again, if that does not work, see important note below.

IMPORTANT NOTE: If layers have been locked using the Keep Layers Locked function, this keyboard shortcut will not work. You will need to edit the Keep Locked layers first.