One at a time so you don't get anything mixed up. Buy the plugs specified in your owner's manual. Some plugs may be difficult to access, and you should make sure the engine is cold. Blow any debris from around each plug with compressed air before you remove it. You may need special wrenches and adapters in addition to standard 3/8 and 1/2 inch spark plug ratchet wrenches. If you have spark plug wires rather than Coil Over Plug (COP), they should be replaced if they've never been replaced. Make sure to carefully remove each wire from the spark plug because they can easily be damaged.
Modern platinum or iridium tipped spark plugs can last 100,000 miles, but after 14 years, if your plugs have never been replaced, they probably should be regardless of the mileage to make sure they can be removed from the engine in the future.