What operating system are you using? On Linux you can create and edit reports under "/usr/lib/python3/dist-packages/OpenNumismat/templates/". On Windows or macOS I imagine that the process would be similar, you just need to find the right folder. You could try using your operating system file browser to
search for the folder "NumizmatPrice", which is a report template bundled with OpenNumismat. That should
help you locate the folder where the reports are. I would advise against modifying the existing report templates as they will be overwritten when you update OpenNumismat to a newer version. Instead, you should make a copy of an existing report template, name it something new, and then modify it to your liking.
The reports are built using HTML and a templating language named "jinja".
More information on jinja:
https://jinja.palletsprojects.com/en/3.1.x/templates/