Can I upgrade or buy a better motherboard that will fit a OptiPlex 755 DT (desktop)

What issues are you having? What improvement are you looking for? Generally it's better with Dell BTX to just get a whole computer that has what you need. Their cases are cut out in the rear to fit each motherboard model, so any chenge there requires metal work. they use proprietary front I/O cables and sockets that change over time. For instance if you wanted a Optiplex 780MB to get DDR3 the front cable pinout is different requiring the cable, and front panel circuit board to be swapped which might not fit. There are metal tabs under all the memory slots, and expansion sockets. If the new board has any difference there you will have metal under live motherboard components. If you could define "Better" I could maybe give more specific advice.
The better motherboards often came in bigger cases which have more expansion slots. so won't fit in your DT format.

BTX Obsolete How?
Click on the image for some ATX-BTX perspective.