Nikon D780 DSLR Camera (Body Only) - Manual (Page 720)

674 Menu Guide > B The Setup Menu
Use [ Lock mirror up for cleaning ] to lock the mirror up so that
dust that cannot be removed using the “clean image sensor”
option can be removed manually. Note, however, that because
the low-pass filter is extremely delicate and easily damaged, we
recommend that manual cleaning be performed only by a
Nikon-authorized service representative. For information on
cleaning the low-pass filter, see “Manual Cleaning” (
0 827).
Lock Mirror Up for Cleaning
G button
U B setup menu