To change the severity of an Issue, go to its details page and then follow these steps:
On the Sidebar of the details page, click the current severity.
This action displays a list with severities available for Issues. The severities you see on the list may be set up depending on what you need for the current project – for more details, you can go to Severities.
Select a new severity from the list.
The severity of the Issue is automatically updated.