I replace my cabin microfilters once a year. I live in the south in high humidity, and it is easy for them to collect mold and mildew. In this (rare) case, I do not buy the OEM parts. The OEM parts are sold under part number 64319159606 and the 4 of them (sold as a set) retail for $131.61 r.
Instead, I buy Mann CUK 22 008-4. I buy them from Amazon.com for $45.68 with free shipping and no sales tax. The CUK 22 008-4 gets you all 4 filters you need to do the change.
The Mann filters are not identical to OEM. The OEM filters have a plastic ring around them. The Mann do not. The Mann have "wings" made of filter material and they actually fit a little bit nicer than the OEM ones. But, do not freak out during the DIY as they are different.
The Mann filters come with a nice DIY guide with pictures. However, I deviated a little from their directions and I will post my way here.
To begin, un-clip the weather stripping at the top of the engine bay.
Once removed, you will see 3 8mm bolts for each filter housing. Remove those. The Mann directions specify you should remove the hood (alarm) switch. I did not. Leave it alone. You can easily turn over the left (as you sit i the driver's seat) filter assembly with the switch still connected. No need to pry and remove it and risk engine bay scratches. Once the 3 bolts are removed and the housings are turned upside down, push the OEM filters out from the inside. Your hand will fit into the inlet. You may need a flat blade screw driver on one of the 2 filters.
As far as I can tell, one of the four filters does nothing. It is there in case the car is RHD. And I assume if you have an RHD car, one of the LHD filters does nothing. You will see the blanking plate sealing off the air flow under one of the 4 filters. I assume BMW uses 4 in order to make them all the same and symmetrical. ???
Replace the filters taking care to install them with the "air flow" indicators on the filters in the proper orientation.
Replace all parts as you removed them.
One way to greatly extend the life of your microfilters is to turn off the compressor (or "Auto" climate control if you use it) a minute or two before you shut off the engine. But, leave the fan blowing. This will help blow out the moist condensation air in the system and will prevent mildew. It works really well and I do it every time I shut down my car.