Since you have to change the username, you have deactivate all accounts in Quicken with this bank.
First be sure to have (or make) a backup of your Quicken data file ... just in case something goes wrong ... Go into Edit Account Details (right-click the account name in the Account Bar), click the Online tab, click Remove from One Step Update. Click the General tab. The Customer ID field should now be editable. Clear it or enter the new customer ID. Click OK to save it. Repeat this process for all other accounts you have with this bank. _ALL_ accounts with this bank must be deactivated for the next step to work. When done, go back to the Online Services tab of one of the accounts. Click Activate One Step Update. Very carefully read all screens and follow prompts. When you get the screen which lists all accounts found at your bank, carefully verify that Quicken is suggesting the right connection between bank account and Quicken account. A correct linkage should say something like: [Bank checking] LINK [MYChecking] If it is not correct, click the ADD / LINK / IGNORE choice/button and choose the correct Quicken account from the list. Repeat this for all other accounts listed on this screen. Proceed. There will be a transaction download as part of this process. Carefully accept downloaded transactions. If older transactions that have previously been downloaded and accepted, re-accept. If in doubt, delete a downloaded transaction before creating a duplicate register transaction.