One of the ways could be to use an extension such as ReReplacer (https://extensions.joomla.org/extension/rereplacer/
). Of course, there could be a performance issue as the HTML output would be searched and replaced with the revised string everytime a page loads.
The URL for the logo file is got from the following statement in the file index.php that is located in the directory \templates\protostar.
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$logo = '<img src="' . htmlspecialchars(JUri::root() . $this->params->get('logoFile'), ENT_QUOTES) . '" alt="' . $sitename . '" />';
Another way could be to create a copy of the template Protostar and edit the file index.php of the copied new template. Of course, you have to make sure that the file index.php of the template Protostar template is changed, the change has to be implemented in the copied new template.
First check whether the https works fine in the site.