Hi @Stan Verhoeckx ,
Per my test, I can update the folder name by PATCH /drives/{drive-id}/items/{item-id}
. Here is the test result
You will need to check the drive id and item id. First you need to get your site id by following api
https://graph.microsoft.com/v1.0/sites/Contoso.sharepoint.com:/sites/testsite?$select=id
And the highlight marked id is site id
Then use the siteid check the drive id
https://graph.microsoft.com/v1.0/sites/siteid/drives
Finally get the itemid by the api, and you can update folder name with drive id and item id now.
https://graph.microsoft.com/v1.0/me/drives/driveid/root/children