Log in to your Fidelity account
Go to Accounts & Trade
Select Account Features
Choose Dividends and Capital Gains
Select the account you want to update
Choose the security or set the dividend election for the account
Select Reinvest in Security or Reinvest in Mutual Fund, if available
Save or confirm your changes
Repeat for each account or security you want to reinvest dividends for
Check that the election shows as active after saving