You can preview the extensions by going to the form or portal URL. You can also click Open Extension (portal with or without login) / Open Form (form with login).
If the Form has no login, click Open Form, then Create Records to preview creating a new record.
To preview editing a record, click Open Form, then Edit Records. A modal will be displayed. Click Test Editing a Record in Table to open a random record or copy and paste the Formula to Edit Records, then add it to a new formula field to your table to edit a particular record.
💡Tip
While your logged-in, you can edit your extension while accessing the form or portal. Just click Edit Extension. This button is not available on end user's view.
On the free plan, you're only allowed to preview the extensions yourself. The extension would look and work to your users how it appears to you when you access its URL while logged in.