If you moved your Experiment ahead a phase before you were ready to, you can always go back! You can click the phase titles in the upper left to visit a different step to add data, but if you want to revert to a previous phase entirely, here's how.
1. In the Experiment screen, click the Three Dot Icon in the top right navigation bar.
2. Click Back to [Plan] Phase.
3. Confirm, and your Experiment will revert to the previous phase. Please note that any changes you made in the former phase will be lost.
NOTE: Completed Experiments cannot be returned to a previous phase.