Clean the exterior of the greenhouse by washing the panes of glass with warm water and a sponge. Higher parts of the greenhouse can be reached by attaching a sponge to the end of a broom handle or a bamboo cane. Old plastic labels or a gentle jet wash attachment on the end of a hose are ideal for getting in under the window panel joints to remove the build-up of dirt underneath. Clean on a moderately windy day and let the breezes dry out the greenhouse as it's washed. Once the panes of glass are dry, insulate the greenhouse with bubblewrap to protect any plants you are overwintering, and to encourage the greenhouse to heat up more rapidly in the spring.
Check the panels for any broken or cracked glass. They should be replaced. If you are planning on doing the job yourself, then you should wear gloves and goggles. Wooden frames can be painted using a wood preservative, making sure that none of the solution touches the plants.