Let me help you go over to your online banking and link your checking account. You can disconnect the bank that's connected to QuickBooks Online so you can link the correct one.
To disconnect an account:
Click Banking, and then select the Banking sub-tab.
Select your savings account, and then click its Edit icon, then Edit account info.
In the Account window, select Disconnect this account on save.
Click on SaveandClose.
Once disconnected, you can link your checking account. The following articles will help guide you in uploading and categorizing your transactions in QuickBooks Online: