Reports allow you to add custom logos, text, header and footer details. This article will provide steps on customizing those existing reports.
If you have trouble following the steps due to missing templates, or missing information on the report text form, let support know.
Do an Advanced Find for Report Templates

If you know the name of the report you are looking for you can filter by name a the report name and the report template name are the same, but you can also leave the filter blank to search all available templates*.
*If the report exists, but there is no Report Template record, you can regenerate one by going to the Settings record, and click Run Workflow - Generate Report Templates.
Open the Report Template for the Report you want to customize.
You can usually ignore anything that says Sub-Report when it comes to customizations/report templates. They exist but that's not usually where you are making changes.

Errors on the report
If you run the report and find that you get an error saying "Report Text 1 is missing" or "Header is missing" then the system is referring to the report texts. Even if you don't plan on customizing the reports, you can create blank records using the steps above to resolve these errors.
Report text updates are not retroactive, but will appear on any future report generations.