If you use a Divio git repo, then nothing at all.
If you use a custom repo hosted on GitHub, GitLab, BitBucket or elsewhere, then you will need to generate a new key and add the new deploy key to your repository as per the instructions.
In order to regenerate the key:
Go into your application and select
Copy this new key to the clipboard and add it to your repository as detailed in the instructions.
Although GitHub have said that keys generated prior to 15th March should continue to work, Divio recommends that you upgrade your keys as soon as it is convenient. This will both eliminate the risk your key suddenly stops working at some point in the future, and provide greater security by way of a better algorithm.