To find it, you will need to:
1. Identify your transmission type: This information is usually found on a sticker on the transmission itself, or in your owner's manual.
2. Consult a repair manual specific to your transmission type: A Haynes or Chilton manual, or a factory service manual, will have diagrams and instructions showing the location of all the solenoids, including the overdrive solenoid. These manuals are invaluable for this type of work. Searching online for "[Your Transmission Type] solenoid location" along with images might give you a visual clue.
3. Prepare for a potentially messy job: Accessing the solenoids usually requires draining the transmission fluid.
Warning: Working on an automatic transmission is complicated. If you're not comfortable with automotive repair, it's strongly recommended to take your vehicle to a qualified mechanic. Incorrectly diagnosing or working on the transmission can lead to significant damage and costly repairs.