You can change the units of measurement of your global stats by navigating to the cog in the home tab and choosing between metric and imperial.

The units shown in your individual swim activities, though, will be shown in metric or imperial based on whether the pool size you select is in yards or metres.  

If you are using a compatible smartwatch for open water swimming connected with the goggles, the units displayed in the goggles will match the units you have chosen in the smart watch.