How do I view release burnup charts?

You can view release burnup charts in the "Burnup" tab:

If the tab is not shown, click the "More" button at the right side of the tabs. Then select the "Release burnup" checkbox:

You can also view the release burnup chart of a release in the popup dialog for the release:

And if you're a stakeholder in a project, the release burnup chart for the ongoing release will also be shown in your "Overview" tab.