To rename a dashboard you will need to start in the desired entity. From here, you will need to hover over the "Dashboards" section head on the left side. Click the gear icon that will display to the right of "Dashboards" as shown below:
After you click the gear icon, a screen with you current dashboards similar to the one below will appear:
Next, find the dashboard you would like to rename, click the dropdown arrow to the right, and click "Rename" as shown above. After you click "Rename" a module similar to the one below will appear. Change the name, click "Rename Dashboard", and you will be on your way.