Hello @Suebatch
If you're using an iPhone you can do it from the App. From the Today Screen in the App, tap on your profile picture in the top left corner, then App Settings, then Units. Tap to change the settings, then sync your Charge 5.
You can also do it from your Dashboard on Fitbit.com if you prefer. Sign in, tap the Cog icon at the top right, tap Settings, then scroll down the page to Units, change, then select Submit at the bottom to save your changes.
I hope that helps a bit.
