Please consult official documentation for your web server to know how to generate a CSR with a 2048-bit key. Most documentation can be found online through a simple Google search. If you use a web hosting service for your website, check with your web hosting support team on how to generate it from their system. Some common scenarios for generating CSR on various server platforms are listed here.
Alternately, if your web server / hosting control panel allows you to import an externally generated CSR & Private key, you can generate a CSR (with a 2048-bit Private key) using our online free service.