Mk120 reverse operation

i’m helping my daughter with a science project and we need the mk120 to work in reverse. the buzzer needs to sound when there is no obstruction to the ir path.
i searched and found an earlier post with a similar question. the answer was to swap pin 9 and 10 of IC1 on the receiver. i don’t understand how to physically do this. sorry for the silly question but i’d appreciate a detailed explanation.

The easiest way of doing this is by interrupting both track at the solderside of the PCB and swapping the connections to the IC socket by means of two pieces of wire.