Just done this today as she had an oil and filter change yesterday. Right hand button ("STEP") till you get to "next service due" and a mileage displayed, then hit the left button ("RESET").
Hi there! It's done with the functions on the overhead display at the front. I can't remember accurately but go into the menu, go along till you get to service interval, then reset to another km / mile range, if you have a handbook it's in there somewhere (probably I think!!!)
Out by Lanark here, Central Scotland. Welcome to the forum and Voyager ownership. The forum (and the other one, just swap .co.uk for .com) is a great source of help, with most niggles you'll come up against already having been solved. Hope you enjoy the car.