How to change the logo
From Online Manual
Many themes provide an easy way to show a custom logo or banner for your site. All you need to do is to upload the image file (you can use cpanel or ftp), note the URL, and tell the theme to use that URL. You may want to test your desired logo or banner on a test version of your site, to see if it is compatible with your theme.
- Upload your logo/banner to your desired location by using ftp or a method provided by your host. A good place to have the file hosted is the root of your forum (or an images folder there). Note where it is, and obtain its URL.
- Login to your forum and go to Admin > Configuration > Current Theme
- In the text input labeled Logo image URL:, enter the URL you copied earlier.
- Select the Save button to save your change.
- Note: for a theme not having the input in step #3 above, go to that theme's support topic for further assistance.