Installing a class
This is a duplicate of the following questions:
- How can I manually install a package on MiKTeX (Windows)
- How to have local package override default package
The basic answer is you should never put your own packages or classes in the main directories of your TeX distribution. You should always put them into a local directory. Consult the accepted answers to these questions for more details of how to do this.
Every own created package and class should go into a local tree, which is supported by all TeX distributions. For MiKTeX read the MiKTeX documentation, section "Register a user-managed TEXMF directory". It shows how a local tree can be created, if not already present.
I recommend to put your class file into a dedicated directory.
After copying the class into your texmf directory structure, refresh TeX's file name database. In MiKTeX, open MiKTeX Options/ Settings and click the button "Refresh FNDB", or run
mktexlsr
(ortexhash
in some distributions) at the command prompt.