The logo that appears on the top of customer facing reports is associated with the "Theme". To change the logo go to Admin - System settings - Themes, then in the list of themes click the Upload button to select the logo file from your local disk.
Unless you have added more themes then there will only be one on the list. This is the one for your default logo.
The logo file should be a maximum of 300 kB and its type .jpg. There should not be any spaces in the file name. After selecting a file it will show "fake path" in file location. This is set by your browser and is not to worry about. Press upload to upload the new logo.
After uploading the file, see how it looks by opening a sales invoice for instance.
Note that company information shown in the footer of customer facing reports is set from the selection of the partner id on this screen. By default this is the partner number 1. So to change your company address etc. then go to company 1 and edit the information there.
If you do not see the new logo on a particular report, this may be because you have a custom report where the logo is embedded directly. Contact our support to resolve this.