Update field on contacts based on value in related opportunity

Hi

I have a field on my contacts ("Have opportunity - courses") which I would like to update with a value ("Yes" or "No") if there is an opportunity, on a specific product category,  related to the specific contact.

So if there is a sales opportunity for the product category "courses" I would like to change the "Have opportinity - courses" on the related contact to "Yes".

I've tried creating a workflow for the "contact" enitty but I can't figure out how to look across entities as I would need to look for data in the "opportunity" entity. Is there a simple way to do this?

Thanks in advance,

Thomas

CRM level - Newbie