How do you replace a T Gauge coupler? Very carefully! The short story is that you just twist the coupler 90 degrees. The video will show how it works in practice. 

Why would you need to replace a coupler? You may want to have slightly longer couplers, change the type or it may be damaged. Another reason could be to remove the tension spring to make it easier for rolling stock to navigate tight curves. 

You can find replacement couplers on 

Replacing Couplers Tutorial