Good afternoon dear friens,
I want to evaluate a HiLo problem in pipe/fiting joints, how could i change the fitting wall thikness in the model?

In CAESAR II, just model the fitting as a distinct element, and set the thickness appropriately.

Make sure you research this against your applicable piping code. CAESAR II will apply the rules from Appendix D (B31.1/B31.3) to determine the SIF. The software will be unaware of your "hi/lo" situation and any special fabrication issues.