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I'm afraid you can't go back to the old layout. Fitbit say:
"After you switch to the new app experience, you can no longer access the old Fitbit app design. Over the next few months, the old Fitbit app design won’t be available, and the new app experience will become permanent."
taken from: Help article: What’s the new Fitbit app experience?
Thanks. It seems the layout is not customisable; as I would like to have calories as the prominent item on dashboard but it's fixed with steps.
Also can't seem to change order of the remaining items.
Fitbit Product Experts Alumni are retired members of the Fitbit Product Expert Program. Learn more
You may want to follow and contribute to the main thread for this change at:
https://community.fitbit.com/t5/Android-App/Fitbit-app-new-Dashboard-Android/td-p/3412588