Hello Jimmydire,
We're sorry for the trouble you had with the Fill&Sign feature. Are you referring to change the font when you choose the 'Sign' option in Fill&Sign?
You can change the Font Style while typing your signature and 4 type of style is available, You can choose from a small selection of signature styles; click Change Style to view a different style:

The fonts cannot be changed in the Fill&Sign as this is a designed behavior. For detailed information, you can refer to Adobe article Fill and sign PDF forms using Adobe Acrobat Fill & Sign
If you have a different query, Feel free to update this discussion for any further assistance. If possible share a screenshot of the error/issue you are facing. To share the screenshot, refer to https://forums.adobe.com/docs/DOC-7043#jive_content_id_How_do_I_attach_a_screenshot
we're here to help, just need more info.
Thanks,
Anand Sri.