You have to think of an A/B test as a newsletter. A newsletter cannot be stopped or modified at the time of sending.
The same goes for A/B tests: has your A/B test been sent? Then it is no longer possible to stop, modify and continue your A/B test.
The only option left to you is to completely delete the A/B test in the newsletter overview by clicking on the three dots and choosing 'remove':
Therefore, test both your Variant A properly and your Variant B properly.
Once sent = sent.