How Do You Remove old autocomplete entries in Messages

My friend has a new phone number. I've updated his contact in contacts, but the old number still shows up when I type his name in messages in High Sierra. Further more, even if I select the new number, messages will simply send it to the previous number, because it's attached to his name, and it's the number I texted to him in all previous conversations. Yeah, that's right, messages isn't even sending the text to the number I've choosen - it just sends to my my cousins name, with the old number, even though that's not saved under his contact anymore. I had to go to my iPhone and manually text him from contacts. Messages on my computer is totally messed up.