METHOD 1 - This is the preferred method
- Follow knowledgebase article 4310893 which details how to open the administration dashboard for one or more agents
 - Click on Settings | Administration
 - Click on Connection Details
 - Go to select the problematic agent
 - Edit the credentials
 - Click on Validate connectivity to verify the new connection details
 - Save the changes
 
METHOD 2
The database and OS userids and passwords can be configured in the individual agent properties.
- These can be accessed through the navigation via: Administration | Agents | Agent Status
 - Edit the Properties for the agent
 - Click on Modify the private properties for this agent
 - Update the User name and Password fields under the "Monitored Connection Profile" area.
 - Click the Save Changes button
 - Deactivate the SQL Server agent
 - Activate the SQL Server agent
 
 
The following video details how to change passwords in SQL Server agents