How do I forward the current environment to a child process with added/removed env variables?

Migrating from Boost 1.85 to 1.88 I am trying to migrate existing code from v1 to v2. Unfortunately the documentation is still scarce and I have trouble converting our code. In one case I want to use the current process environment and forward it to a child process with some variables added and some removed. I haven't found anything in the documentation on how to do this and also looking into the header files of environment.hpp left me wondering if this is possible at all. Please let me know how to do this.