I found how to rename cost1 cost2 etc.. but how about renaming the cost column? how can i do it?

Double click on the column heading and change the Title. You can only rename custom fields, but you can change the title of any field to anything you like.

thanks, that was easy...

You're welcome -- but remember Microsoft defines what is included in the Cost field. So you can title it "Labor Costs" but it is total cost including fixed, material, and cost resources.