Cannot find any instructions on how to do it.
I want to update from v2.31 to the latest portable version.

I think it's enough to copy new executables from the archive file to the old version Tixati_portable catalog.

This is how to update portable Tixati: Download the latest version, decompress it (unzip/untar), and copy the files of the new version to the directory (folder) of the old version, overwriting the old files. Then start/execute Tixati's new version executable file (.exe) from the directory/folder of the old version.