I must point out that here you are upgrading from a stable 1.7.4 release to a unstable development Release Candidate. some things are broken like structures!
The procedure consists of basically a backup, extract the tar over your current files, run tiki-install.php login as admin, run the tiki1.7-1.8 installer
Then you need to modify the php file called structure1.7-1.8.php and enter your database logon to it
surf to it only once! then delete it!
then using phpMyAdmin or something similar run the 2 comments fixes in the order of 1 first then 2 on your tiki database.
As you can see the upgrade is a "dirty" process at the moment and will be worked on ready for the official 1.8 release.
But that would get you up and running, current problems are structures cannot be created, this has been fixed already for RC4 and also structures dont import very well. and probably other things with it being a RC and not a stable release.