The Primary Fuel Filter is the Filter on the Engine, the secondary is a separate from the motor. The primary is small filter (usually) Some folks only run a primary.
The secondary is either a fuel only or a fuel/water separator filter. A Racor Fuel/Water Separator has a clear plastic bowl screwed underneath a cartridge fuel filter. The fuel filter looks like a car oil filter except it has an open top and bottom. The function of the secondary filter is to separate out the big chunks of crud and any water in the fuel. Water is heavier than the fuel so it collects in the clear bowl at the bottom of the assembly. Once the fuel is "cleaned" of the big junk and water, the primary, on your motor, polishes it before it goes into the injectors. The Secondary may have a 30 micron rating, whereas your primary may have 10 or 20 micron.
As far as the method that you need to do change your filters, its boat specific. However, it is best to remove all of the air from each filter first instead of doing a full system bleed. If you don't get the air out of each filter assembly first, it may take a awhile to push it all out. This why I recommend that you fill each filter as much as you can with new fuel before you reassemble. Reduces the amount air that you need to bleed.
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