Saks Fifth Avenue will expand and modernize its stores across the nation over the next five years as part of a $300-million program announced by the New York-based retailer. Saks Chairman and Chief Executive Melvin Jacobs said more than 25 stores will be renovated as part of the project. In addition, new stores will be opened in Denver; Portland, Ore.; Minneapolis; North Palm Beach, Fla., and Tysons Corner, Va., bringing the total number of Saks stores to 49. The company's flagship Fifth Avenue store will be expanded to increase selling space by about 30% by 1989. Saks is owned by the British company B.A.T. Industries.