Something along the lines of - Incorrect mode selected ■■:■■ to ■■:■■, mode should be rest.
You can write it wherever you like, normally it is the rear of the chart but if you do it on the front make sure it doesn’t cover any of the traces. If you go over the water best not to do it on the front of the chart.
Yes, dont frett over it mate, anybody can make a mistake. Make a note on the printout (presume its a didgi) and keep a copy for your own records if you so wish.