Erase and Install (by @user68186)
When you start the fresh install of Ubuntu 19.04, you should see an option to erase everything and install. This will remove all the partitions and create new ones spanning the whole 2TB.
This option will delete everything in that disk! That means Windows, Ubuntu 18.10, any data partition, any recovery partition etc. etc.
Alternative option
Alternatively you could boot a live system (eg. Ubuntu 19.04's install media), and using gparted to delete your windows partition, then 'grow' your 18.10 partition to use the whole disk as you'd like, then do your install from that media.
You can opt to install using 'something-else' and ensure 'format' checkbox is not checked. This takes note of additional apps you've installed in your 18.10, erases system directories, installs, then re-adds apps you added (if in repositories). It doesn't touch your user data (unless you accidentally selected format).
Of course don't forget to backup any data you don't want to lose, and please note this will be slower than erase/install!
19.04's coming EOL
Ubuntu 19.04 won't receive updates after that EOL in late Jan-2020, but a few days after Ubuntu 19.10's release, you'll be offered the chance to release-upgrade to 19.10 (or you can do it yourself any time after this until shortly after 19.04's EOL).
This is a consolidation of comments by @user68186 and myself
