The code B12 for a Honda Pilot indicates three different services that are needed. The B indicates that there is an oil and filter change required. The 1 is for a tire rotation and the 2 is for the replacement of the air filter and cabin filter, as well as the drive belt needs to be checked.
Changing an air filter in a 2009 Honda Pilot is simple to do. First raise the hood and find the air filter box. There will be clips holding the box closed, remove the clips and open the box to reveal the old filter. Replace it with a new unit.