How often should the transmission fluid be changed in a 1994 Volvo 940 with 157000 miles?

According to Volvo's recommended maintenance schedule, the transmission fluid should be changed every 30,000 miles or every 24 months, whichever comes first. However, since your vehicle has already exceeded the recommended mileage, it would be advisable to have it checked by a qualified technician to ensure that it is still at a usable condition.