To access them, you would need to remove the transmission. This is a significant undertaking requiring specialized tools and knowledge of automatic transmission repair. It's not a job for a novice mechanic.
If you're experiencing shifting problems, it's best to take your vehicle to a qualified transmission specialist or mechanic. They can diagnose the problem correctly and determine if a solenoid (or other transmission component) needs replacement. Trying to locate and replace them yourself without experience is highly discouraged.