Once an account is established as a Joint account, it cannot be split into an individual account. Instead you will need to close the Joint account, then open an individual account.
For help closing your cash account, follow the steps outlined here.
Please note that either owner of the joint account can close the account on which they are a joint owner, and removing a joint owner does not remove them from your dashboard.
To remove a user from your Personal Capital dashboard, please see How do I Remove a Joint Owner From My Dashboard?.