The stable version of fitdistrplus can be installed from CRAN using:

install.packages("fitdistrplus")

The development version of fitdistrplus can be installed from GitHub (remotes needed):

if (!requireNamespace("remotes", quietly = TRUE))
   install.packages("remotes")
   
remotes::install_github("aursiber/fitdistrplus")

Finally load the package in your current R session with the following R command:

library(fitdistrplus)