To switch a logo, you first have to examine the source code of the respective template. Graphics and images can be defined in the HTML and in the CSS structure. The size of the graphics is usually harmonized for the template.
If you want to exchange one of the images, you can do so by various methods.

Method 1
1. Create a graphic in a resolution and a file size that fits the space.
2. Load the graphic into Joomla!’s Media Manager.
3. Click on the graphic to get a link to it.
4. Change the respective <img src= > tag in the source code of your template to the new image.

Method 2
1. Create the new graphic and give it the same name as the graphic in your template.
2. Simply overwrite the old graphic with the new one.