If you have Released onboarding in Prod, conducted the test, and decided to conclude it, click "..." next to the onboarding and select "Finish test".
A pop-up will appear, prompting you to "Please, select Onboarding which you want to publish instead of this A/B test."
In the dropdown list, you can choose:
An onboarding that participates in the current A/B test
Any created in the current project.
Clicking On Yes the AB test will be stopped and selected onboarding will be shown to your users instead of AB tests
The article with Questions and Answers about AB tests you can find here