Adjusting the back straps on your mask, if they are too long for your head, is extremely simple.
the operation must be done symmetrically on both belts
Turn the buckle inwards
Lift the latch that blocks the belt
take off the leather strap
cut the strap just enough to adjust the mask (use a scissors or a suitable cutter)
re-thread the belt in the retainer through the back of the neck
tighten by pressing so concluding the operation.
Repeat everything on the other belt.