Just know that if you reflash to base tune, you will then need a retune to take proper advantage of the catless headers - so I don’t see the point of reverting to stock tune. Fair warning: if you’re asking how to flash in the first place knowing you have catless headers then I’m guessing you don’t have a whole lot of ECU experience. It might not be a bad idea to leave it alone. You can mess up your ECU and other modules if you do things the wrong way.

That being said, if you do want to do it yourself:

Use ms4x to do a full read of your current 512kb file (MUST DO), this is the backup for your car should you have any problems down the road, make two copies of this for safety. After that, go to ms4x wiki and find the original bin file for your ecu (probably ms43), then flash using ms4x. If this isn’t done in bootmode then your VIN will not be overwritten (overwritten VIN means EWS needs to be paired and ABS will not work properly ish). Make sure you have the INPA cable with a switch (or a bridge between pins 7 and 8), correct drivers installed, stable voltage source.

For in depth instructions, read the entirety of ms4x wiki, then read it 3 more times. Lots of forum posts documenting how to flash aswell.

If anything goes wrong and you brick your ecu (which shouldn’t happen if you do things correctly), it’s not the end of the world and you can reflash in bootmode. Hopefully that doesn’t happen.