Code P0715 showing on my vehicle . Took to Mitzi told internal trans prob help??

Mitzi had my vehicle for almost 3 hours and said they ran diagnostics on it and they thought the transmission speed sensor was bad. This was the second time I took it in to have it checked. I cannot keep driving the vehicle the way it is as I don't want to do any further damage to the trans. I was quoted a price of $1800 (parts and labor) for a used trans with 93k miles with no guarantee this would correct the problem. Not willing to do this at this time and especially for a trans with so much mileage. Can get one on internet with less mileage if I decide to change it. If it is the transmission speed sensor, why can't we change it ourselves. Need to know where to find it on trans. Also, read where internal trans problem was one of the most common misdiagnose of the P0715 code. I don't want to replace the trans if the problem is the trans speed sensor. Trans does not make noise only shifts roughly. Unplugged TCM overnight and problem was corrected for a month or so but started up again.