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How do I go about editing the irb120_3_58.xacro or urdf file to make this change?

You don't.

Those files are not meant to be edited.

Instead, you should create a new .xacro file yourself, which combines your end-effector model and the base robot model.

Such a composite xacro is a recurring construct which you can also use to create other structures (such as entire work cells).

For a simple example, you could take a look at Working with ROS-Industrial Robot Support Packages: Example: adding an end-effector to a robot.