To edit an existing chart, you must first know if it is a shared dashboard and, if so, you must have Shared dashboard admin rights. A shared dashboard will have the standard shared icon after the name of the chart. 

If you are not a Shared dashboard administrator, you will not be able to edit shared dashboards directly.

However, you will be able to create your own dashboard by copying the shared dashboard. A personal dashboard that isn't shared will not have the shared icon next to the name.

To edit any chart, click on the cog wheel to the right of the individual chart name.

This will take you to the set-up page of the chart, and from there you can make any edits.

Each chart type has a different set of parameters you can edit, as well as universal ones. You can look through each chart type in this section and see how to set them up and edit them.