Go to season mode and then the rosters section. Create a player with good stats like a good shooter or keeper. Put the player on your team using the edit lines screen(on the rosters screen), the stats you have given that player will make your teams 'overall' stat go up and hey presto a better team overall.
Team ratings
North American All-Stars: 99
Canada: 97
Colorado Avalanche: 97
USA: 97
Sweden: 96
Detroit Red Wings: 95
Dallas Stars: 94
Philadelphia Flyers: 94
World All-Stars: 94
St. Louis Blues: 93
Czech Republic: 92
New Jersey Devils: 91
Russia: 88
Toronto Maple Leafs: 88
Los Angeles Kings: 87
Phoenix Coyotes : 87
Pittsburgh Penguins: 87
Buffalo Sabres: 86
New York Rangers: 86
Ottawa Senators: 86
Washington Capitals: 86
Finland: 85
Florida Panthers: 85
San Jose Sharks: 85
Anaheim Mighty Ducks: 84
Edmonton Oilers: 84
Boston Bruins: 83
Carolina Hurricanes: 83
Chicago Blackhawks: 83
Germany: 82
Vancouver Canucks: 81
Calgary Flames: 79
Montreal Canadians: 79
New York Islanders: 79
Slovakia: 78
Columbus Blue Jackets: 74
Tampa Bay Lightning: 74
Nashville Predators: 71
Minnesota Wild: 70
Atlanta Thrashers: 69