AWK: merge two files and replace some fields

Shell Programming and Scripting
1

Need some code tweak:

awk 'END {
for (i=1; i<=n; i++)
  if (f2[_])
    print f2[_]
}
NR == FNR {
f2[_[n = FNR] = $1] = $0
next
}
$1 in f2 { 
delete f2[$1]
}1' FS=, OFS=, 2.csv 1.csv > 3.csv

file 1.csv have:

$1,$2,$3,$4,$5,$6,$7,$8,$9......

file 2.csv have:

$1,$2,$3,$4,$5,$6

files merges fine, but I need replace file 1.csv values of fields $4,$5,$6 with values $4,$5,$6 from file 2.csv, other fields from $7... must be untouched.

2 
awk 'NR==FNR{a[$1]=$4;b[$1]=$5;c[$1]=$6;next} $1 in a {$4=a[$1];$5=b[$1];$6=c[$1]}1' FS=, OFS=, 2.csv 1.csv
3

Thanks, but not work.