Yes, you can amend your return to correct line 6a reporting your Social Security benefits if you have already filed and your return has been accepted.
If there is not an amount being reported on line 6b of your return and you need to change line 6a to a lower number, then you should not need to amend your return. The change would not change your bottom line for the tax return.
Take a look at the following TurboTax help article for instructions to amend your return: