I use a wrench or plier type tool and squeeze the pad against the caliper to depress the pistons.
I can only get the rear by hand the front is too hard for me so I use the above and then they slide out
13 E92 M3 ZCP
Stock w/Rs3 - WSIR 1:33:9 / BW C13 2:03:6 / ACS 1:56:7
KW CS w/Rs3 - WSIR 1:32:8 / BW C13 1:59:2 / ACS 1:55:1
APR Wing/Rs3 - WSIR 1:30:9 / BW C13 1:57:7
-No Power Mods