To rename the chart, you need to open it in editing mode. Click on 'Analytics' in the left menu. All your charts should be displayed in the Analytics menu in the 'My Charts' section, then click on the pencil icon:
The chart name is available in the top bar above the chart visualisation and you can just type in the cell to update the name, then click on 'Save chart'.
If the chart is included in a dashboard, the name will be updated there too. If the dashboard is shared with other users, all users will have the updated chart name in their version of the dashboard.