Transmission not shifting properly, where is the speed turbine shift speed sensor located?

When driving when it accelerate to 4th gear a lot of times it will downshift itself into 3rd gear, eventually it will shift. It has done it when I had it in drive instead of over drive it will shift down into 2nd gear, I put it on obd2 scanner and speed turbine shift speed sensor code comes up. Where is that located and is it easy to fix? Thanks in advance.