You can try it with :normal! command alone:
:%normal! 05f,dF,0Pm`0x``P
Which executes normal commands for all lines (%) one by one:
0 to go to beginning of the line
5f, to go to 5th comma
dF, to cut back till comma
0 to the beginning
P to Paste before current char
m` to mark end of pasted text
0 to go to beginning of the line where now there is a comma
x to cut a comma
`` to go to the end of the pasted text we marked earlier
P to paste a comma
Note, it wouldn't work for a well formed csv with "fields that have, commas".
There are probably better ways to do it too :)
PS, you can replicate normal command just typing it on the csv line 05f,dF,0Pm`0x``P
PPS, tested with:
1,2,3,4,5sdf skljsf,6,7
1,2,3,4,5 sdkj lskdj ,6,7
1,2,3,4,5 aksldjfl sjdf ,6,7
result:
5sdf skljsf,1,2,3,4,6,7
5 sdkj lskdj ,1,2,3,4,6,7
5 aksldjfl sjdf ,1,2,3,4,6,7