Target Corp. is trying to win skeptical shoppers back by giving them a year to return purchases of private-label brands such as Merona shirts, Threshold book shelves and Room Essentials towels. The move is meant to show confidence in the brands, which make up about a third of Target's $72 billion in annual sales, after the retailer cut back on quality during the recession to get prices low enough to appeal to an increasingly frugal shopper.