On the Casio website, you will find quick guides for Casio Calculators including the ClassWiz.
Casio Classwiz quick guide
You can find an excellent calculator guide for the Casio FX991EX-ClassWiz on Dr Frost’s site.
For the complete manual from Casio see this page, note the calculator is listed under fx-570EX / fx-911EX on the Casio manuals page. Select this page for the direct link.
From The Calculator Guide on YouTube see this comprehensive playlist on the Casio Classwiz fx-99EX. For example see this introductory video on using Statistics mode to find the mean, variance and other statistical summary data.
Other examples include further statistical videos such as Normal Distribution calculations, integration and differential calculations and using Complex mode.
See also this playlist from Casio on the Classwiz FX 991EX.