We do not a have a supported way to edit the favicon on the Formstack-hosted forms. This may be something that we will add support for in the future but in the meantime, we have a few workarounds that will allow you to achieve the desired favicon changes:
1) Advanced HTML
If you have a strong understanding of HTML or have a development team on staff you can find the Advanced HTML Header and Footer editing options by going to the form > Build > Theme. The Advanced Options editor is the option located in the bottom section of the page. We strongly recommend that only those familiar with HTML make these adjustments.
2) Professional Services
If you do not have the time or internal resources to make these HTML edits on your own, our in-house Professional Services Team can coordinate with you on these changes for a designated fee based on the changes you're looking to make. To get in touch with our Professional Services Team on this, please submit your request here and we'll get right back to you!
3) Embedding the Form