See the big lug with a recess having a bolt head inside of it? The lug is next to the tensioner. The tensioner is that cylindrical thing with a rubber sheath below the idler pulley. Use a ratchet wrench with an appropriate sized socket to turn that bolt head inside the recess to compress the tensioner so you can loosen the belt. That takes care of the belt for the AC compressor. The second belt drives the power steering and the alternator. A similar lug and an idler pulley can be seen between the fan blades in the picture.
You can replace the two belts in five minutes or less I think.
'01 540it, 6/01
'03 325i 5 speed, 9/02
'10 535ix. 9/09
'10 mini 6 speed
'15 mini countryman 6 speed