A lot depends on how the boards are installed. The lateral (across the trailer ) boards are often harder to replace. The boards that run the long way of the trailer are easier, especially if they don't fit in a slot at the bumper end of the trailer.
If the bolts are rusted in, and most of the time they will be, they need to be cut off. From underneath the trailer! That means a lot of hard work, lying on your back, unless you have access to a lift.
Some, especially the lateral boards, actually require cutting the metal strips holding the boards in place with a torch to replace them, and then welding the strips back in place after the new boards are inserted.
It is usually a pretty labor intensive job. I've gotten too old and fat to attempt it! But if you are young, agile, good with tools, and have a lot of time on your hands, you can do it yourself.