[Samsung Gear S3] BiggPharma:
This is the method I used to change the CSC code on my SM-R760 to an Australian code with a view to setting up Samsung Pay. It took a little hunting around the forums to find all of the information, plus I ran into a few things while I was doing it that didn't seemed to be mentioned elsewhere so I thought I would summarise it all below for us noobs.

All credit to the experts, in particular [MENTION=1030585]adfree[/MENTION]

WHAT YOU WILL NEED:

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NetOdin3

https://xdaforums.com/gear-s3/development/rom-samsung-gear-s3-sm-r760-t3588062

Combination Firmware

https://xdaforums.com/showpost.php?p=74600026&postcount=26

Your current stock firmware (in my instance, R760XXU2BQC5)

https://xdaforums.com/gear-s3/development/rom-samsung-gear-s3-sm-r760-t3588062

Your CSC code

https://xdaforums.com/showthread.php?t=787163

A PC with a WiFi connection

INSTRUCTIONS:

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1. Check reactivation lock is off

- On your phone, go to Samsung Gear App

- Go to Setttings Tab, choose "Find my Gear"

- Make sure Reactivation Lock is off

2. Connect your watch to your PC

- Hold down the home/power button until you see the "rebooting" screen

- Release the home/power button then quickly press it three times to bring up the boot menu

- Quick press the home/power button to cycle through the menu to "Download (wireless)"

- Long press the home/button to confirm the menu selection

- The phone will then go into WPS mode

- Double press the home/power button to change to AP mode

- On your PC, enable WiFi and connect to the access point on you watch.

NOTE: Until you successfully connect to NetOdin, your watch may display red text stating "Connection Timeout" - this is OK

- Open up NetOdin. You should now be connected to your watch (you should see the IP in the NetOdin window and "Connected to xx.xx.xx.xx" on your watch face.

NOTE: I had to disable Windows Defender public firewall temporarily for NetOdin to work properly

3. Flash combination firmware

- Load the unzipped .tar.md5 file into the "AP" box of NetOdin, wait for it to finish checking then click "Start". This should take about 10 minutes to complete after clicking "Start".

NOTE: I had left "Automatic reboot" ticked in Odin, so once it completed the watch restarted. No issue, except had to go through step 2 to reconnect the watch to the PC before proceeding to the next step

4. Flash CSC file from stock firmware

- Load the unzipped CSC_xxx.tar.md5 file into the "CSC" box of NetOdin, wait for it to finish checking then click "Start". This should take about 1-2 minutes to complete after clicking "Start".

5. Factory reset

- In your Gear, go to Settings > Gear Info > Reset Gear > Confirm

NOTE: I got a warning at this stage that Reactivation Lock was on. Don't know why I got this message as I had definitely turned it off in step 1. I ignored it (no guts, no glory!) and was fine. Your mileage may vary

6. Set CSC code

- In the Gear, go to the Tizen Keystring app

NOTE: screenshots of what your interface will look like https://xdaforums.com/showpost.php?p=74591475&postcount=13)

- Type in the following code exactly: *#272*719434266344#

- You will be taken to a list of CSC codes that you can scroll through. Find the code you want to change to and select it. I used "XSA".

7. Reflash complete current stock ROM

- Load the unzipped CSC_xxx.tar.md5 file into the "CSC" box of NetOdin, wait for it to finish checking.

- Load the unzipped AP_xxx.tar.md5 file into the "AP" box of NetOdin, wait for it to finish checking.

- Load the unzipped BL_xxx.tar.md5 file into the "BL" box of NetOdin, wait for it to finish checking.

- Click "Start". This should take about 15 minutes to complete after clicking "OK".

You are done!

After this, my Gear picked up the OTA update to Tizen 3.0.0.1

So after all of this effort, NAB doesn't currently support Samsung Pay, so I can't 100% confirm this will work, but everything I've read says it should.