You can send a PUT request to /api/issues/{projectname} with an _id and one or more fields to update. On success, the updated_on field should be updated, and returned should be { result: 'successfully updated', '_id': _id }.
this is a very vague description. What exactly you fail to do?
Obviously you want to handle a PUT request which includes an _id and undefined number of additional fields. You could prolly use a find and update mongoose query, providing the _id to look for and any additional parameters as arguments to be updated. If the query succeed without error and return a valid document, you should send the specified info object { result: 'successfully updated', '_id': _id }
When all else fails, look at the source for the tests on github.
Your PUT route appears to mostly work, but the test still fails. Instead of believing your PUT route, the fCC tests also GET the issue by id to verify the response from PUT. Your GET route does not return issues when requested by id (it returns all issues, not the one issue), so your GET route is causing your PUT route test to fail.
So, implement GET by id (that would be one of the one filter GET requests…).