You want a post mount to post mount +20 adapter, like
this one from Shimano. There are others from other brands, any +20mm most mount adapter will do.
What IS mount adapter you need for the rear depends on the OEM size of the rear rotor. The mount
will look like this, but you can't always trust the description. If the bike "natively" takes a 140mm rear rotor, you would need a 180mm adapter (essentially a +20) to mount a 160mm rotor.