Deploying
Click Deploy on a service card. The pop-up shows the commit that is running and checks the branch for a newer one; pick which to deploy. With auto-deploy on (in Settings → Source), every push to the branch deploys by itself.
History
The Deployments tab lists every change to what runs, newest first: deploys, settings saves, reverts and failed builds. Each revision shows the commit it runs. Use the filter to show only Git updates.
Reverting
Next to an earlier revision, click Revert. The service goes back to that revision's code and settings; this creates a new revision, so you can always move forward again.
Build output
The Pipelines page shows every run with its build output. If a deploy fails, the output usually says why — see A deploy failed.