Open Mendix 9.12.1 and choose your project
then it will ask you if you want to convert it from 9.12 to 9.12.1, choose covert-in place.
You just need to open the project in Mendix 9.12.1. This will ask you if you want to convert it, which is what you want to do.
Once upgraded, you won’t be able to downgrade it again.
Hope this helps.