Go to the JIRA ticket from the link in the spreadsheet.
Add yourself as a watcher for the issue in JIRA so that you receive any updates (e.g., see additional comments, be notified of status changes). To start watching an issue, click on the eye icon in the upper right corner.
Read through the ticket, including the comments, so that you understand the issue.
Unless there is a specific server listed for testing, use https://test.archivesspace.org to test the issue (admin/admin for the staff interface login).
If the issue appears to be fixed, leave a comment saying as much.
If the issue has not been fixed, leave a comment as well, including as much detail about the behavior you are observing. Screenshots and/or an explanation of how to reproduce what you found are very helpful additions.