Presuming you don't have a digital speedo - those have an E/M switch to make the change.
On the analog dash, there is a single wire connector near the steering column (under the dash), it will have a blue or green wire on one side, and a black wire on the other. You may have to remove the plastic plate beneath the column to see it. When it's disconnected the readout will be English. When it's connected the FDC and CCP switch to metric. Strangely, Cadillac didn't even include this connector in the FSM schematics - it's one of those little secrets you only learn about at this forum.