By calling the company, visiting the local branch (for banks or building society), or by visiting their website, where they might have an online form that you can complete.
If you know the account details you can use the free websites Settld or LifeLedger to help you contact the utility companies, banks, pension providers, insurance firms, TV and broadband providers and even social media websites about a bereavement.
Doing this should help save time and stress as you only need to upload the death certificate once within the service.
You can also contact a number of financial institutions, including most major banks and building societies, even if you didn’t know about the account, using the free online Death Notification Service
You should also place a deceased estates notice in The Gazette to inform any other creditors. If you don't do this, you might be liable for any unidentified debts.