Yes, the severity of an incident can be changed at any time, provided the incident status is not Closed.
Follow these steps
Step 1:
From the left navigation menu, click on Incidents.
Step 2:
Locate the incident for which you want to change the severity status.
Step 3:
Click on the Edit icon under the Actions section for the respective incident.
Step 4:
The incident details will open. Locate the Severity option and select the required severity.
Step 5:
Scroll down and click on Submit from the bottom-right corner.