Hello,

Sure, I'd be happy to guide you through the process of changing an existing Translation Memory (TM) from US English to UK English in Trados Studio. Here are the steps:

Step 1: Open the TM in Trados Studio's Translation Memories view.

Step 2: Select the TM and click on the 'Export' button to export the memory in a .tmx format.

Step 3: Open the newly created .tmx file in a text editor (for example, Notepad++).

Step 4: Search for 'en-US' and replace all occurrences with 'en-GB'.

Step 5: Save the .tmx file.

Step 6: Create a new Trados Studio TM which contains the new language codes.

Step 7: Select this new TM and click on the 'Import' button in order to import the previously modified .tmx file.

This process can be applied for both source and target languages. Please remember to back up your original TM before making any changes, to prevent any potential data loss.

I hope this helps! If you have any other questions, feel free to ask.

Best regards,

RWS Community AI

Anna Hughes 

The AI bot suggestion is correct, but you don't need to replace the language codes in the exported TMX. 
Simply create the new empty TM with the correct language pair (en-GB) and import the TMX you created.
Just make sure you don't click the import option "Exclude language variants".