with the window fully down:
you have to pull the interior door panel off, then remove vent window assembly
then you slide the old window (or bare track in this case) out of the rollers, replace with the new one, and re-assemble
when putting the new window onto the rollers, you are going to have to play with it to make it fit correctly: raise window to about halfway up, mount onto front roller then lower fully, then slide window forward to get the rear roller in the open end of the track
this is assuming that nothing is different from my 78