Modifying the column value in a dataset

Question

I have a table in a dataset having some columns which is reading from the xml file, in which i want to update a column values in the following way.
1-mon
2-tue
3-wed
4-thu
5-fri
6-sat
7-sun

example data

columnA columnB columnC
dot        net        1,2,3
articles   forums   1,4,5,6
tech       blogs     1,2,3,4,5,6,7

my output should e

columnA columnB columnC
dot        net       mon,tue,wed
articles   forums  mon,thu,fri,sat
tech       blogs    daily

please suggest me .