Is there a way to modify a weighing scale so it can show a certain number when being used? I am a beginner at modifying things so please be kind.

 

The scale probably use load cells to determine the weight. These cells will probably be configured in a "bridge" configuration and the output of this bridge will be fed in an amplifier (most likely the square integrated circuit). The output of this amplifier will be used to determine the weight of the person standing on it.
If you were to modify this circuit I would probably find the output of this amplifier and hijack it and provide your own voltage. As in the example shown below. Here I added a potentiometer, but you could also provide your own circuit.
Hope this gets you going, good luck!

 

Ooh okay i see thanks

 

The scale probably use load cells to determine the weight. These cells will probably be configured in a "bridge" configuration and the output of this bridge will be fed in an amplifier (most likely the square integrated circuit). The output of this amplifier will be used to determine the weight of the person standing on it.
If you were to modify this circuit I would probably find the output of this amplifier and hijack it and provide your own voltage. As in the example shown below. Here I added a potentiometer, but you could also provide your own circuit.
Hope this gets you going, good luck!

View attachment 241227

Can i use a 10k OHM preset potentiometer?

 

10k ohm will probably be fine.
To confirm that this is actually how this design works, you could probe the output pins of the amplifier when you're standing on it to see if the voltage actually varies with the weight you are applying to the scale.