Hello Eliza,
It is defined as the variable $data_dir in
tpp_gui.pl. The best way to change it for your installation is by adding the following line to the configuration file
tpp_gui_config.pl , anywhere under the line that says "# SET TPP DEFAULTS HERE" :
'data_dir' => "/full/path/to/data/",
where you will want to use your actual path inside the quotes. Make sure the values are in quotes, and notice the trailing comma. Moreover, changes made to this configuration file will not be over-written when you upgrade to new versions of the TPP. (Though it is always a good idea to keep a backup, just in case.)
Cheers,
--Luis