Upgrading a ZENworks Asset Management/Inventory server from 7 to 7.5 is pretty straightforward. However, there are a couple of things to watch out for during the install.
The ISO image found here is used to do both the upgrade and fresh install for both Asset Management and Asset Inventory.
You can grab the PRUs (Product Recognition Updates) online, or download them local and point at them in the filesystem.
The same works for applying the latest ZAM patch, (IR3 at the time of this writing).
If during the upgrade the web interface will not allow you to login, and it looks like the old style interface, then you may need to re-deploy the Tomcat WAR file that the Web interface is run from.
Unpacking and running a newly deployed WAR really can take a few minutes, even on fast hardware, but it should not take the 15 minutes that the documentation suggests.
Once it is done, you should see the newly designed login box on the Web Console URL, and be able to login.
If during install you run into a 15116 error when doing the database installation step, it is usually because your Group Policy requires an SA password for the MS SQL 2005 Express password that is more complex than the one being used. Apply a less restrictive password policy and try the install again.