Hi
The short answer:
Open the classic client, click on tools, object designer, go to the reports section, i assume you want report number 206 - Sales - Invoice - but make sure that this is really the one
Highlight it and export so that if you dont get it right you can restore the original
Click design, if you get a permission error they you user permissions or your client licence dos not permit this and you need to contact your partner
If it opens click View Layout,
The long Answer:
Modifying reports is a development task, i would recommend that you look at completing the developer course if you want to be able to customize your own reports,
Most Microsoft training centers can help you here
You could also chat to your partner to find out if they offer a training course covering reports development
Cheers
Nev