Is it possible to edit (modify) text inside Loaded Family (Titleblock) by using Dynamo?
yes, it’s possible if it’s a parametric and by changing the respective parameter.
You need it to be a label not text and linked to shared parameter. Which is shared by the project and the family. If you Google these terms ‘revit label’ and ‘shared parameters’ you should get more information about how to achieve this. But it is completely possible.
You could also have Dynamo open th efamily document and make the desired modification if there is a non-parameter change, such as a font to update.