You can do a "manual update" - i.e. overflying a known point (a VOR)...
That was the first form of update that existed on early models.
xxx
A couple of years later, started to have capability of tuning a DME and be updated.
So in the mid-1970s until mid-1990s, no DME = NO update (over the ocean).
And since some 15 years ago, GPS is now used, instead of DME update. Works everywhere.
xxx
Hope that answers your questions...
Happy contrails
P.S.
Warning - DME updates - i.e. France.
Numerous DME not co-located with VOR (are not "VORTACs")
Be careful entering DME (TACAN) coordinates, not the associated VOR...
Danger of updating and entering some 5 or 10 NM error at some locations.