Are you trying to use reduced speed mode?  If so, just set a configurable input to be reduced speed and when the input turns on the robot will drop speed, when the input turns off normal speed will resume.

If you want to control it yourself you will need to send to the server the speed commands for controlling the speed slider.  You can find more information here about how to write a function for controlling the speed slider from within the robot program.

Best thing to do is to download the user manual from the UR website here, since I don't know if you have an e-series or a CB3 that is the start of the download section.

It's rather straightforward, before you can make any of those changes you will need to add a safety password in the setup screen.

Don’t have to use a safety plane but you can. You can make a configurable input be reduced mode as well so that the robot goes to reduced mode anytime that input turns on, or offf, I can’t remember which it is.