A cheap and easy way to cover a sectional sofa is to drape blankets and throws over it. You can strategically place them to cover any worn spots or stains and make your sectional sofa look as good as new.
A more expensive way of covering your sectional sofa is to have a slipcover custom made to fit the pieces. The advantage of this method is that custom slipcovers conform perfectly to the dimensions of the sectional sofa pieces and won't be prone to slipping off like blankets and throws. Choose the pattern and color of the material and create a look for your sectional sofa that will make it look entirely different.
Upholstery may be more effective than a custom slipcover depending on the style and make of your sectional sofa. If it has exposed wood arms or bases, upholstery will maintain its original structure and style.
While custom slipcovers can and upholstery can both be quite expensive, off-the-shelf slipcovers can create the same effect, if you can find some that are the perfect size for your sectional sofa. Because these are made in large numbers, you benefit from economies of scale and can buy them for a fraction of what you would spend to have slipcovers custom made. Your choice of patterns and colors are limited by what is made available in stores.