library(dplyr)
library(magrittr)
set.seed(234)
df<-data.frame(ano=sample(2011:2015,7,TRUE),mes=sample(1:12,7,TRUE),semana=sample(1:5,7,TRUE), vlcasa=sample(100:1000,7,TRUE),vlcarro=sample(1:100,7,TRUE))

df %<>%
 mutate(vlcasa=case_when(
      .$ano == 2011 & .$mes == 8 & .$semana== 2 ~ 200,
      TRUE ~ as.numeric(as.character(.$vlcasa))
  ),
      vlcarro=case_when(
        .$ano == 2011 & .$mes == 8 & .$semana == 2 ~ 9,
        TRUE ~ as.numeric(as.character(.$vlcarro))
      )
  )