You can access the 'Reset and Resync' button by going to the Dashboard and tap on 'Account Sync' at the top.
You will be brought this page:
You will see 'Reset and Resync' at the bottom of the page.
Reset and resync will clear all your local changes and reset the app to the state you would find your App if you logged in and performed an initial sync.
Sometimes, a reset and resync is required to take advantage of new features in the App. When this happens, we will always alert you in-app.
To be sure that you will not lose any data created, always tap on the large green "Sync" button first to send your local changes BEFORE selecting reset and resync (otherwise you would lose the changes made).