I'm wondering what can I do to make my children receive better genes?
For example, can fitness, cold showering, studying etc. change my hereditary material and affect my children?
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It only takes a minute to sign up.Sign up to join this community
No amount of lifestyle practice will change your germline (heritable) genome.
However there were 2 recent articles where researchers experimentally fixed dangerous genetic defects in embryos using CRISPR-Cas9 which could form the basis of future tech that corrects all known dangerous genetic diseases.
Bear in mind that the idea of "designer humans" (using gene-editing to make more perfect humans rather than just fixing serious hereditary problems) remains incredibly controversial and bitterly contested/debated (and illegal in most Western nations). Expect it to remain that way over the next 10 years.
If you want perfect offspring and aren't up to waiting, you might consider diving into robotics and AI instead and build a robot. That way, if it has any problems, you can always order new parts or provide tech support.