You can change the BPM manually using the available options from the Zulu's main interface. Luckily, the developers created FAQ pages to help users with various issues they might have.
To change the BPM manually, read the instructions on the following page: http://www.nch.com.au/dj/kb/1226.html
And it's generally recommended to consult the Help section from within the application in order to learn about the features present in the software.