How to update custom field in JIRA via REST API?

This (resolution description) custom filed should be updated while the "resolution" changed to "fixed" so I used the "transition" REST API call to do it.i have passes the following jason string via REST API call.but no sign of working as expected. can't I change those two fields with "transition" REST API call?

{"update": {"resolution description": [{ "add": { "body": ""+ errors +"" }}]},"transition": { "id": "5" }}

"errors" is a variable with hard-coded value.