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 Tgauge.com
My T Gauge Journey 
I received a starter set with the EMD GP8 from TGauge.com. Unfortunately, the loco did not have a coupler at either end. TGauge.com did send me a coupler set for free to install myself. Luckily, I found this video. Very, very helpful and informative. While trying to install the coupler, I managed to pop the truck off the chassis somehow. The coupler is installed into the truck but I think I managed to break whatever holds the truck onto the chassis. I can see the pip in the center of the truck and the indentation in the chassis but there doesn’t seem to be anything to hold the truck in place anymore. I’d love to see a video of how to remove and reinstall the trucks onto the chassis. In fact, it might actually be easier to remove the trucks, install the couplers onto the trucks and then reinstall the trucks onto the chassis. Any help would be greatly appreciated.
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Check the link below, it’s another post. I think that is what you are looking for. But to access the tension springs on a GP8 you need to remove the body shell first. The chassis for the GP8 is the same as in the video about the tension springs.
https://tgauge.home.blog/2020/12/27/replacing-bogie-tension-springs/
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