Very simply, backup your database through phpMyAdmin, their documentation will give you details on how to go about that.
Then Download the 1.9.1 tar or zip file and FTP upload it, making sure it uploads correctly.
Now simply browse to tiki-install.php login with your admin password and run the sql upgrade scripts.
Its really a case of merging the FTP install guide with that of the UpgradeTo19 guide.
Hope it helps