Before diving into solutions, it's important to identify the source of the dog smell. Is it coming from the surface of the couch, or has it seeped into the upholstery? This will help determine the best course of action to take.
Start by thoroughly cleaning the couch. Remove any removable cushion covers and throw them in the wash with a cup of white vinegar. For the main body of the couch, use a mixture of water and a pet-friendly, enzyme-based cleaner. Be sure to follow the manufacturer's instructions and test in an inconspicuous area first.
After cleaning, sprinkle baking soda over the entire couch, focusing on areas where the smell is most prominent. Let it sit for a few hours or overnight to allow the baking soda to absorb the odors. Then, vacuum the baking soda off the couch using a brush attachment.
If the weather permits, take the couch outside to air out for a few hours. Fresh air can do wonders in helping to eliminate any lingering odors. If the couch is too large to move, open windows and use fans to circulate air around the room.
If the dog smell persists, it may be time to consider professional upholstery cleaning. A professional cleaner will have the tools and expertise to thoroughly remove odors from the couch, leaving it smelling fresh and clean once again.