There are four different processes, how the software can be read from the control unit and be rewritten:
Variant 1 soldering ( Replace the chip in the engine control unit):
The ECU have to be removed from the car and the chip will be removed (unsoldered). The original software is then read using an EPROM programmer. During the software is optimized by a chip tuner, a socket can be soldered. This has the advantage, that you can replace the chip simpel at any time without soldering. The tuned software is stored on a new chip - the original chip remains unchanged and can be re-installed into the socket anytime.
Variant 2 OBD (read / write via the service interface ):
Via the diagnostic interface of the vehicle, the software can be read/write with an appropriate tuning tool to the ECU. This method requires neither mechanical tasks nor soldering.
Variant 3 BDM:
In this method soldering is also not required, but the engine control unit must be removed. Inside the ECU a standard BDM interface can be found, which allows a read or write process of the software.
Variant 4 Tricore:
Similar to the BDM procedure, the internal interface of the engine control unit is also used to read / write the software. Here it is also necessary to solder 3-5 pins to perform the tuning.