If you only want to do a computation on one or a few columns you can use transform
or simply do index it manually:
# with transfrom:
df <- data.frame(A = 1:10, B = 1:10)
df <- transform(df, A = A*1000)
# Manually:
df <- data.frame(A = 1:10, B = 1:10)
df$A <- df$A * 1000