You should buy the software from here https://www.statease.com/shop/design-expert/. However, if you want the free software, I suggest for you to use R programming with RSM Library.
The R programming installation can be acquired here: https://cran.rstudio.com/
After you installed R, you can install its user interface, called "Rstudio" here : https://posit.co/download/rstudio-desktop/
Then you can install the RSM library in Rstudio, by puting this code:
"install.packages(rsm)". After you install the library, you can switch on the library by puting this code "library(rsm)".
The guidance for using RSM in Rprogramming can be found here : https://cran.r-project.org/package=rsm