Just changing the oil is pretty straight forward but in order to do it right, you really should drain, disassemble, inspect the components, replace the seals and dust boots, reassemble, and refill. there are some really good videos on youtube . It also kinda depends on how happy you are with the stock suspension and whether or not you want to upgrade to a cartridge style spring set up. It is pretty straight forward, but you do need a couple of special tools to do it right. If you don't own an impact wrench (either electric or air) and aren't interested in owning one, I's suggest finding a good Indy shop and seeing what they charge.
To tackle it yourself you'll need the aforementioned impact wrench plus
a 2.5 inch 6mm impact hex driver
An ignition alignment tool (you can try to make one with a 9/16 brass tube, but you can also buy one for 35.00 on any of the bike part stores online)
A good bench vise and fork tube holder- https://www.ebay.com/itm/Motorcycle-...4AAOSwovNaMuGi
and a 1-5/16th socket ( for the fork tube caps
A fork seal rebuild kit
Fork tube seal driver - Posse Fork Seal Driver
Good motorcycle jack
H-D service manual for your year and model bike
You can buy all of this stuff for way under 4 bills and have the satisfaction of doing this yourself. Do it once and see how easy it really is!
Here's a thought, if the dealer is going to charge you 4 bills to replace the fork oil, why not spent a couple hundy and upgrade to a cartridge system - https://www.ebay.com/itm/Progressive...MAAOSwnbZYEmAi
Buy the tools and do it yourself?