I would like to change the API owner/publisher of an API I have published, so I can keep publishing new versions of this API with another user. I am using wso2 v2.6
I've been looking for an option in the Publisher & Store UI but did not see the option
This is not possible as the owner (provider) is one of the fundamental attributes of an API. Even if you change the owner, you would not be able to create new versions as this would be considered as a new API, not a version of the previous API and this would result in duplicate context. The same validation prevents you from creating two separate APIs with the same context.
If you want to change the owner of the API by deleting the previous one, you can do so using the cli tool[1]. Here you can export the API, delete the previous API and reimport the same API with a different owner (provider). Please note that cli tool is only available with the WSO2 WUM updates from July 8, 2019 and you need WSO2 subscription.
[1] - https://docs.wso2.com/display/AM260/Initializing+projects+using+the+CLI+Tool