On the Carousel INS, DME updating required you to enter the DME update area of the INS and enter the lat/long and freq of the DME to be used. I vaguely remember that it required you to simultaneously push 7 & 9 to enter DME mode. Two different ones, ie one in each INS, gave quicker results if available. This system gave an excellent update. You did NOT press DME update because there is no such button/switch on the Delco units, at least those fitted to the B747 classic. Pushing "hold" as you overflew a VOR gave a poor update because you never really knew when you were over the actual VOR transmitter due to the size of the cone at crz altitudes.