How to stop uno from running program when powered on?
The simplest thing is to upload the new program before connecting the hardware.

If this isn't practical, connect a jumper wire from the Reset pin to ground (Gnd) before powering it on. This will hold in a reset state, and the program won't run.

Then, compile and upload your program. As it starts to upload remove the reset wire. That should upload it without running any existing program.