Some old benefits, like income-related Employment and Support Allowance (ESA) are ending. People on those benefits will need to move to Universal Credit to keep receiving benefits.
The Department for Work and Pensions are sending out letters to people who are affected by this change. This letter tells you when and how to apply for Universal Credit.
Mencap has also got some information on the process and what you'll need to do once you receive a letter, take a look at the website here: mencap.org.uk/help-and-advi...
There's also an easy read guide you can share, and details of where you can get extra support if you need it as well.