Be very careful with this. It's possible to render the whole ECU unbootable (yes, it's basically an embedded microcomputer) at worst.
Manufacturers release code updates for their cars over time. A generic re-flash will stand out like a sore thumb if the car goes in for diagnostics at any point. For example the engine number and chassis number will not match your car. This could even cause problems with security systems such as transponder keys etc.
A custom remap (which is set up for your car exclusively - not even two identical models will have identical code) is by far a much better option.
Good tuners will look at the sense (diagnostic) data on your ECU and produce a control set that more closely matches your car. Diagnostic data is not just for faults! This method provides a far better drive, in my opinion, with all the power and torque 'wrinkles' smoothed out long before any performance increase is applied. You can also get some things set to your own liking, throttle pedal response curves etc.
As a side note, I would advise that you completely avoid plug in tuning boxes, even the most costly are very crude.
Incidentally, what car are you planning to tune? And, B - welcome to Torque Cars.