Revoked the API key you were using? Worry not, because you can simply create a new key and update the connection. Do NOT delete the exchange connection as doing that will stop importing new transactions.
To update a connection with a new API key:
Open the app and go to 'Settings'.
Open 'Connections' (or click here) and open the connection you want to update.
Enter the new API Key and Secret and tap 'Update connection'. Voila!
PRO TIP: You will not be able to update some of the connection settings but you can update the API key/secret.
PRO TIP 2: The connection will begin importing the new transactions and it will not need to reimport the old transactions, as long as you followed the steps above.