There are basically two ways to do this. 1. Open the tmx in Notepad, locate "srclang=" in the header and change the language code in the quotation marks, i.e. replace the current source language with the target language that you want to become the source language. Then reimport to Memsource.
2. It is also possible to use a third-party tmx editor, such as Olifant. Here you need to export the TM as tmx, open the tmx, select the corresponding source and target in the top bar and save. Then reimport to Memsource.