At its core, the 'upgrade' consists of nothing more than unzipping into an existing Tix directory - after you've logged out and copied yr Tix directory someplace as fallback/backup.
The potential gotcha is in the unzip software you may use. (WinZip is OK.) That needs to do a file-level unzip, and not a directory-level unzip.
What this means is that file
'incs/mysql.inc.php' holds the keys to your existing Tix database, and that file
must be protected at all costs against a deletion. (If you're ftp-ing newly unzipped stuff, then you'll need to ensure you protect the noted file against any over-write.)
Note that any db schema update gets done automatically at the first login after the above.