How to change the temporary directory of HellaNZB through LottaNZB?

It's currently not possible to do that directly using LottaNZB, but you can change it manually.

Shut down LottaNZB (so that HellaNZB doesn't run anymore either).

All temporary files are stored in /home/you/.local/share/hellanzb (or /home/you/.hellanzb in older versions). You can move this "hellanzb" directory whereever you want (e.g. to another partition).

Then, you need to fix the HellaNZB configuration file managed by LottaNZB so that it also points to the new location. Open /home/you/.config/lottanzb/hellanzb.conf. Replace every occurrence of "/home/you/.local/share/hellanzb" with the new path. After having saved the file, you can restart LottaNZB.

Note that you can make hidden folders and files (whose names start with a dot) visible using Ctrl-H.