Hi Michael and welcome to our forum!
It depends on what you mean by "form". Sometimes we use form to mean the XML and sometimes we use it to mean the XSN. If you are talking about instances, and the logo was added as an image to the XSN, you can change the XSN and it will update for all forms. If you have the same logo in multipe XSNs, you should change it to be a link to a domain resource and then update the image pointed to by the link. If you are attaching a different image to every XML instance, you won't be able to easily update this (although you could use a bulk upload tool like ours to modify more than one XML file at once). Cheers
Edit the XSN that the forms are using. You can find this by saving the forms
Patrick Halstead
Project Manager at Qdabra