Here’s the link to submit the documents. If you click through, it’ll show what documents you can submit for your circumstances.
https://www.gov.uk/repaying-your-studen ... nt-details
The information you need to give online is the same as what you would send through the post so it’s easier to do it online.
If you haven’t added your email address to your online student loan page, might be worth doing as these emails alert you to when you need to do something, rather than having to wait for the letters which are pretty slow.
Good luck!