Hi, i want do unsend all messages, that i send to one contact. I dont want to delete whole conversation. only unsend, cause when i delete he can also see what i send in the past.
AFAIK, there’s no other method than manually unsending one by one.
If you use a software, request this feature to the developers & maybe they can implement it.
Hacking something up in Python/NodeJS with either selenium pur Puppeteer might be the way to go 
If that is outside your skill set, maybe @dimitri is able to do it for you (with proper compensation I hope)