Step 1: Select on the app you want to update and click on 'VERSION OR PLATFORM'
Step 2: A pop-up will appear enter the version of the updated app you want to submit (Optional: version must match the version you put it the xCode)
Step 3: Enter an information on what's new on this version and click 'save'
Step 4: Choose a build then click 'save'. (new build must be uploaded from xCode or other programs)
Step 5: Click 'Submit for Review' when everything is all set. (wait for 4-7 days for beta reviewers to check the app, you can set it to manually or automatically put the app live on store when review process is done)