Spodman, as BOAC and BIK 116.8 say, I think you're confusing INS and FMS (or FMC).
It's many years since I've used INS and memory grows dim
but I'm pretty sure that there was no automatic update facility. We could update it manually, but this involved inputting the co-ordinates of a known point and then hitting the "update" button as we passed overhead. In practice, this meant using the co-ords of a VOR and updating when we hit the cone of silence. Bearing in mind the size of the cone at FL350 this didn't produce a particularly accurate fix (within a mile or 2 at best), so unless the INS had gone walkabout in a big way (and ours very rarely did) it wasn't of too much use.
On the other hand, FMS is a computer which accepts inputs from many sources, including INS or IRS, VOR, DME and GPS. If in range of radio navaids it will update itself automatically and continuously using DME/DME or, if 2 DMEs not available, then VOR/DME, or if no DME at all then VOR/VOR. There are restrictions regarding the angles between the aircraft and the naviads etc, but off hand I don't know what they are - if you think about it, I'm sure you'll see the logic. If out of range of radio navaids, then GPS, if installed, is the primary source of updating. There is no facility (on the one I use anyway) to update manually, but with all the above there's no need to. There's also no indication of when the last update was, since it's almost continuous anyway, but there is an estimate of the accuracy of the current position. The only time we see the nav accuracy wander away from "extremely blimmin' accurate"
is if we are out of range radio navaids. (ie over the ocean, or somewhere like Siberia/Northern Canada) and there is no GPS installed (or it's U/S).
Hope the above is of use (and that it's accurate!!
)