Raised bed gardening solves many problems for both experienced and novice gardeners. Soil inadequacies, pest control, drainage issues and even short planting seasons are all favorably addressed by installation of raised beds. If your raised beds are bordered in wood, however, that wood will eventually rot. Even if you use naturally rot-resistant wood, such as cedar or redwood, it will eventually require tending due to rot. Correcting the issue will depend primarily on how badly damaged your raised beds have become.
Diagnose the problem. If only one or two planks are so rotten that they are no longer doing their job, that is considerably easier to fix than an entirely unsalvageable raised bed.
Remove soil from the affected area of the raised bed with a shovel. Use the wheelbarrow to hold the soil. If you have to remove a large amount of soil, use the wheelbarrow to pile the soil in an area of your yard that is out of the way while you work.
Unscrew the rotten planks. Measure them, then use the measurements to cut a new plank to replace the old plank. Mark the measurements on the new plank of wood with a pencil. A jigsaw or a miter saw and miter box are your best bets for cutting planks.
Sand any rough portions of the new planks that will be exposed. Line the new planks up with the wood to which they will be attached, then mark where the existing holes meet the new planks. Drill pilot holes where you have marked to prevent the wood from splitting, then screw the new planks in place of the old ones using galvanized screws.
Finish the new planks of wood with a wood treatment, unless you are using naturally rot-resistant wood such as cedar or redwood. If you are growing edible plants in your raised bed, use a non-toxic wood treatment such as tung or linseed oil. These will help to protect your wood against the elements.
Allow the wood to rest for the amount of time specified by whichever treatment you have applied to it. Different products require different waiting times before they have cured enough to stand up to the elements. Replace the original soil after this amount of time has passed.