@Forest It really depends on how you are using the agent once created with chrome extension.
If you are on hosted app : Go to bottom of your agent page and click on the edit button will open the below dialog box with complete agent configuration, where you can change anything you want. Each field contains the following JSON object which is self explanatory by property name.
If you are on desktop app : Open the scraping agent in notepad and you may change the value, property you want.
If you want to fully change the selectors or fields, you can re-create the scraping agent in chrome and then copy paste the configuration from chrome to your existing agent.
To see the agent config in chrome app : Click on the option button > go to agent tab
You can also paste your existing agent here to import and then change the configuration in chrome.