Remove all trash, wet items, floor mats, and seat covers
Vacuum carpets, seats, and crevices thoroughly
Dry out the car completely with doors and windows open
Use fans or a dehumidifier to speed up drying
Check for and fix any water leaks
Clean all affected surfaces with an upholstery-safe cleaner
Treat carpets and fabric with a mildew remover
Wipe hard surfaces with a vinegar-water solution
Sprinkle baking soda on carpets and seats, then vacuum it up after several hours
Replace the cabin air filter
Clean the air vents and HVAC system
Run the heater and air conditioning to help remove moisture
Use activated charcoal or odor absorbers inside the car
Keep the car parked in direct sunlight when possible
Repeat cleaning if the smell remains
Seek professional detailing for severe mildew or mold
Avoid leaving damp items inside the car