When you run the tiki-install in the 1.8 for the first time you will get asked for the database conenction details.
It would be very wise to duplicate your 1.7.5 database to another database, for example called tiki18
Then connect the tiki2 site to the tiki18 database and follow the upgrade instructions within the installer.
Dont run this upgrade on your live main data without doing a backup first. The upgrade to 1.8 is not as easy as previous versions -> UpgradeTo18