Edit 'Welcome Mr xyz' message on Contribution Form
I think you're looking for the "Welcome Mr X (Not Mr X, or want to do this for a different person?)" message.  That text is in templates/CRM/Contribute/Form/Contribution/Main.tpl

It is possible to override templates - see https://docs.civicrm.org/dev/en/latest/framework/templates/customizing/ for details.  As the documentation warns, there are problems associated with doing this.