Replacing a bad 750 brushed motor is nothing rocket science. It works just like a regular 550 except for the bigger size.
Before doing the replacement, first connect the motor wires to a new motor and see if this is truly a motor issue. If the ESC runs fine with a new motor, then it is pretty sure the existing motor is bad and must be replaced.
You need to carefully remove the water hose to the cooling coil, then remove the entire motor mount. From there, take out the cooling coil and the shaft cup.
Attach the new motor to the motor mount after the cooling coil is with the motor. Then secure the shaft cup (usually there is a hex screw). Note that the motor shaft on the new motor must have a D cut for the hex screw to hold it properly.
Almost all 750 motors have internal cooling fan blades. When the motor is overheated usually it is those blades that start to melt first (you will see smoke coming out from the motor).