One of the reasons we use Danish Oil on our products is how easy it is to refinish or repair.
General refinishing is easy. Simply sand the area you'd like to refinish using 150 then 220 or finer grit paper. It isn't always necessary to sand all the way down to the bare wood, unless there is wax present or you are applying a different color. Any wax must be completely removed before applying new finish. Once the old finish is sufficiently-removed, apply the stain of your choosing per its application instructions.
To repair a small stain blemish (like a water mark), use a fine sandpaper or steel wool on the affected area. Make sure that there is no wax in the area. Wipe off any dust, and apply more oil of the same color per the application instructions. Wipe off the excess and allow it to cure.
Minor dents require repair to the wood and the finish. If the grains are not torn, gently sand the finish down in the affected area using a fine sandpaper or steel wool. Apply some water to the damaged area and allow it to soak in. Take a damp cloth and apply the tip of a hot iron to the cloth right over the dented area. Be careful not to burn your wood or to apply heat to other areas. The goal is to convert the water in the affected area into steam. This will expand the wood grain in the area, perhaps even reversing the dent (causing it to bulge out slightly). Allow the area to dry completely, then sand and refinish.
Scratches and deep dents or gouges are another matter, because they usually have torn grains. There is simply no repairing or refinishing an area with torn grains; it will darken dramatically relative to the surrounding wood as soon as new stain is applied. If you really want to, you can use special paints, pens or fillers to cover over the damaged area and refinish.
Repairs to the upholstery are often best done by simply removing and completely replacing the old upholstery. This requires simple tools and inexpensive hardware -- a tack remover, tack hammer and more tacks -- or can be done at your local upholstery shop.