This can be done with str_replace_all from the stringr package.
S <- "My name : Harry, Age : 23, Address : London,"
library(stringr)
str_replace_all(S, ":[^,]+,", ": X,")
#> [1] "My name : X, Age : X, Address : X,"
Created on 2019-06-25 by the reprex package (v0.2.1)