That's a bit tricky. Normally, you would download the installer file (a compressed EXE file) and install the base program. If you log into your account, you can download the installer file to another computer, move it to a flash drive, and then to your own offline computer (don't run the installer, save it as an EXE file and then copy the EXE file to the new computer.)
However, you have to update the software several times during the tax season, because the base software does not contain the final forms and instructions. At a minimum, you need to update once in January (around January 15-31to get most of the final forms, and at least one more time before you file, to check for last minute changes.
Normally, the base software will go online and download the update, but the update size is basically the full program (200MB or so), so if you have a slow connection, that will be a pretty slow process.
If you look in your online account under your purchases, you will see where to download the EXE installer file. I'm pretty sure that file gets updated when new updates are released. So that creates a second path to get an updated version, which would be to download the EXE file on the faster connection, copy that to a flash drive, take it to your offline computer, and then uninstall the previous version manually and install from the new EXE file (which should contain the update).
It would still be worth attempting to install the base version from a flash drive and then trying to download and install the updates over your slow connection. It might not be as slow as you think, and it would be easier than the manual method.
