If you’re the only one signer, you can remove the signature and then work on the document or edit the source document.
To remove your signature, right-click the signature and then choose Clear Signature. However as there is one more signer involved, there isn't much that can be done once the PDF becomes read only.
For more information, please check the help page https://helpx.adobe.com/in/acrobat/kb/edit-signed-PDF.html
Regards
Amal