In this experiment, you can safely change it directly in the pynn code (brain editor).
But fyi, this "Please remove all references to the population in the transfer functions and try again. " popup arises when you try to delete a population from the graphical population editor and you still reference it in one of your transfer functions.
The other one "Cannot map parameter 'sensors' in transfer function 'grab_image" means you are referencing in your transfer function grab_image a popultation that does not exist because you renamed it in the population editor or deleted it.
To revert your changes, you can reset the neural network or the whole simulation.