x<-data.frame(matrix(data=rep("2",12),ncol=6))
y<-data.frame(matrix(data=rep("3",12),ncol=6))
z<-data.frame(matrix(data=rep("4",12),ncol=6))
a<-list(x,y,z)

> a
[[1]]
  X1 X2 X3 X4 X5 X6
1  2  2  2  2  2  2
2  2  2  2  2  2  2

[[2]]
  X1 X2 X3 X4 X5 X6
1  3  3  3  3  3  3
2  3  3  3  3  3  3

[[3]]
  X1 X2 X3 X4 X5 X6
1  4  4  4  4  4  4
2  4  4  4  4  4  4

>a
[[1]]
  X1 Column2 Column3 Column4 Column5 Column6
1  2       2       2       2       2       2
2  2       2       2       2       2       2

[[2]]
  X1 Column2 Column3 Column4 Column5 Column6
1  3       3       3       3       3       3
2  3       3       3       3       3       3

[[3]]
  X1 X2 X3 X4 X5 X6
1  4  4  4  4  4  4
2  4  4  4  4  4  4

x<-data.frame(matrix(data=rep("2",12),ncol=6))
y<-data.frame(matrix(data=rep("3",12),ncol=6))
z<-data.frame(matrix(data=rep("4",12),ncol=6))
a<-list(x,y,z)

data_frames_to_change<-c("x","y")
library("data.table")

for (i in 1:length(data_frames_to_change)){
setnames(eval(as.name(data_frames_to_change[i])),colnames(eval(as.name(data_frames_to_change[i]))[2:6]),c("Column2","Column3","Column4","Column5","Column6"))
}

a<-list(x,y,z)

a[1:2] <- lapply(a[1:2], function(thisdf) {
    names(thisdf)[(length(thisdf)-4):length(thisdf)] <- paste0('Column',2:6)
    thisdf
})

> a
[[1]]
  X1 Column2 Column3 Column4 Column5 Column6
1  2       2       2       2       2       2
2  2       2       2       2       2       2

[[2]]
  X1 Column2 Column3 Column4 Column5 Column6
1  3       3       3       3       3       3
2  3       3       3       3       3       3

[[3]]
  X1 X2 X3 X4 X5 X6
1  4  4  4  4  4  4
2  4  4  4  4  4  4

colnames(a[[1]])<- c("X1","col2","col3","col4","col5","col6")