It depends on the date of joining in TCS.
If it is early, then during document verification, you can ask for some time for showing your final marksheet. Utilize this time to clear the backlog exam and get the revised marksheet as soon as possible.
If the joining date is late(5–6 months) then you can easily study and clear the backlog exam and join with an all clear marksheet.
People at TCS understand that you are a fresher and no one does look for opportunities to kick you out. Go at your own pace. :)