By A Web Design
Log in to the PrestaShop admin section as follows:
In your Browser address bar type in your_website_URL/youradminfoldername
Eg.
your_website_URL/admin1234
Enter the Email Address and password provided on the last configuration page when installing PrestaShop, click on Login as shown in Diagram 1.

Diagram 1
After logging in to the backend, PrestaShop Control Panel page opens as shown in Diagram 2.

Diagram 2
Navigate to the Preferences Menu item in the Top Menu as shown in Diagram 3.

Diagram 3
On click of Preferences Tab, the page as shown in Diagram 4 is displayed in the Browser. Locate the link Appearance as shown in Diagram 4 and click it.

Diagram 4
On click of the Appearance link, the page as shown in Diagram 5 is displayed in the Browser.
NOTE: Only the Appearance section of the page that is displayed in the Browser when the Appearance link is clicked is displayed in Diagram 5. This is because the page is very large vertically.

Diagram 5
Click the Browse button from the Header Logo section.
A file browser opens up.
Navigate to the folder on the hard disk where your header logo resides.
NOTE: The Header Logo file must be in .jpg file format. The good news is the header file upload system in PrestaShop converts other image file formats to the .jpg file format. Eg. .png -> .jpg
Select the header logo file and click on Save as shown in Diagram 6.

Diagram 6
On click of the Save, the page as shown in Diagram 7 is displayed in the Browser.
Header Logo, Mail Logo and Invoice Logo change together.
NOTE: If required separate logos for Mail and Invoice can be chosen and saved to the system using exactly the same steps described above.

Diagram 7
Click the Browse button from the Favicon section as shown in Diagram 7.
A file browser opens up.
Navigate to the folder on the hard disk where your website favicon resides.
Select the favicon file and click on Save button as shown in Diagram 8.

Diagram 8
On click of the Save, the page as shown in Diagram 9 is displayed in the Browser.

Diagram 9
Job Completed.
As you can see the favicon and the Header Logo has been changed as shown in Diagram 10

Diagram 10