Chg.var.df <- function(data, prod, vars, chgvalue){

  data[data$Prod == prod, vars] <- chgvalue
  return(data)

}

df <-   read.table(text="   PROD    P1SEG1  P1SEG2
                 1  1   0
                 2  0   0
                 3  0   0
                 1  1   0
                 2  0   0
                 3  0   0   ", header=T)