I would like to change  SWAP PRODUCT to CHANGE IMAGE.

Which file am I looking in as I can't find it anywhere.

Thanks Jimi

Thanks Johnny. Worked a treat. 

I had tried that file before but made no difference so I changed it back.

This time I cleared my cache and it picked up the changes.

Cheers Jimi

Then you'll want to edit the lang.json.

I forgot to say. I'm using the Jquery version..

No need to edit files. You can just set the text via the labels settings (modules area).