In prestashop we do not need to change the logo in the file. We can easily manage through admin side

1) Login at admin side

2) Go to Preference > Themes

3) In current theme we can see different Tab to change the logo for site, email, invoice, Mobile

Logo Change

Logo Change

4) click on Browse button and choose your logo and save the configuration

Now Refresh the page in front. We can see, Our logo is changed.