For example, if you create a workout your machine starts at 1 MPH, because that's the default. But how to set it to start at say, 3 MPH?
Easy, just switch from map view to graph view:
That button switches from the Google Maps interface to a chart that allows you to visualize and control effort:
You can't change the elevation, this is fixed due to the geography that you selected. But you can drag the yellow (speed line) to change the speed of your device.
What if you want to have segments at different speeds?
Easy, just click and drag and it will break the line, and you can drag each segment of the line independently:
As you can see, my route is now broken into three segments, each at a different speed. I don't know if this works with all devices, but it definitely works with my Proform CST-505.