###### Generic ###########
my_df %>% 
  mutate(x1 = replace_na(x1,mean(x1, na.rm = TRUE)))

####### Simba specific ############
data %>%
group_by(                                     # Replace missing values with group mean
    Variables) %>% 
  mutate(
    Values = replace_na(Values, mean(Values, na.rm = TRUE))
  )