Ok ... first you cannot clear the 2018 information so your only option if you want to continue using the same account is to NOT transfer the prior year information when prompted. The ONLINE version will not be available until sometime in December.
When 2019 Online opens.....
There is a tricky way to clear out transferred info in the Online version. But doing this will also clear out all your entries. Go to Tax Tools > Tools > Transfer from your computer > Continue > then Continue without transferring.
Or you can just start a brand new account with a new username.
What did you use last year? You might be able to transfer from that. How to transfer into Turbo Tax Online