You can do this with the API. The API call is an HTTP PUT to /api/v3/selenium/TESTID
You pass along the action of set_score and a score value of pass or fail.
Here is an example using CURL on unix.
Mark a test as PASSED
curl --user USER:AUTHKEY \
-X PUT -d "action=set_score" -d "score=pass" \
http://app.crossbrowsertesting.com/api/v3/selenium/TESTID
Mark a test as FAILED
curl --user USER:AUTHKEY \
-X PUT -d "action=set_score" -d "score=fail" \
http://app.crossbrowsertesting.com/api/v3/selenium/TESTID