Changes to my ucas application and how to tell universities?

One of my alevel subjects I am sitting next summer has changed along with the centre I am sitting these subjects at. I would like to know how I would tell the admissions teams at the 2 unis I applied to being UCL and imperial, where with imperial they have my application and I was invited to sit their admissions test already on Feb 2nd 2024 so should I just email/ phone the departments about this directly?

Have you filled in the form on the UCAS website with the changes? That needs to be done first

No. Also where is this form?