Monarch Money has recently improved how we connect to Capital One and Charles Schwab accounts so that they are more stable and reliable. If you have added your account prior to February 2021, your first step is to disconnect your existing account.
Next, add a synced account for your Capital One account as you would any other. When you click Capital One or Charles Schwab, it will take you to the menu shown below. Click "Continue".
Once you remove your old account you will lose any existing transaction history associated with the old account, and when you add the account a new transaction history will load.
When importing a new account it is impossible to know how comprehensive your transaction history will be once imported. If you choose to leave the old account while also adding a new one (not recommended), account balances and transactions will be duplicated and incorrect.