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If you have central air and heat, you'll need to find the main unit that conditions and circulates the air. This is usually in a basement, closet, or other out-of-the-way space. Try following any visible pipes or ducts through your house until it leads you to the HVAC unit, which will be a large metal contraption. On a HVAC unit, this will probably be somewhere between the air handler, a large box where the air is conditioned, and the plenum, another metal box located next to the unit itself. On a window air conditioner, the filter slot should be behind the grill that faces into your house. The location of the filter slot can vary from one unit to the next, but it should look like a long, thin opening with a removable cover. If you can't find a filter slot on your HVAC unit, try looking in the vents that blow air into your rooms. Some systems have filters in the return air vents themselves, although this is usually only true of systems with 2 or fewer vents. To avoid hurting the machinery or yourself, you'll want the unit to be off when you change the air filter. This will also keep any dust or particles from getting into the machinery while the filter is removed. The filter should slide out easily. Be careful when removing it, as it may be covered in dirt and dust that could come loose and make a mess if handled too roughly. If the filter doesn't come out easily, you can attach a folded tab of duct tape to the side of the new air filter to make an easy-pull handle for next time. The old filter can be thrown in the trash. You may want to carry it to the trash in a garbage bag to keep it from shedding dust and dirt on the way. It should slide in the empty slot with little to no resistance. The filter should have an arrow on the side that indicates the direction of the airflow. Make sure this arrow points in the same direction the old filter did, which will be towards the ductwork and the unit's blower. If you notice any gaps or other signs that the filter does not fit correctly, remove it and put the old filter back in temporarily. You may have the wrong size of filter, in which case you should double check which kind you should have and buy a new one. It's also possible that your new filter was somehow damaged and misshapen. Look at it closely to see if you can detect any damage, and if so, return it to the store and buy another. If any dust or dirt was shaken off the old filter when it was removed, be sure to clean it up. Use a rag or vacuum to remove as much of the dust as possible. It should run the same as it did before, or else have slightly better air pressure. If anything sounds or smells strange, or if the system isn't running at all, call a HVAC technician. This will help you keep track of how often you're changing the filter, and ensure that you remember when it's due for a check. Make a note with your phone or computer, or write it down somewhere that will be easy to keep track of.

Summary:
Locate your HVAC unit if you have central air. Look for an air filter slot in your unit. Turn your unit off. Remove the old filter. Insert the new air filter. Check for any gaps around the filter once it's in. Clean off any residual dust with a rag or vacuum. Turn the unit on to make sure it's still running well. Make a note of when you replaced the filter.