The extension you filed for the 2018 tax return is only for tax year 2018. There are no extensions for tax year 2019 or tax returns for tax year 2019 until the end of the 2019 tax year after 12/31/2019.
If you have already filed the 2018 tax return, received the 2018 tax refund or paid the 2018 taxes owed, and you need to make changes to the filed 2018 return, then file an amended tax return.
Any tax data changed on the amended 2018 tax return where there is a carry-over to the 2019 return should be reflected on the 2019 return when it is filed in 2020.
