$site=Get-SPSite "http://servername:1111/Sites/a"
$web=$site.RootWeb
$contributePermissionLevel=$web.RoleDefinitions["Contribute"]
$contributePermissionLevel.BasePermissions="ViewListItems, OpenItems, ViewVersions, ManagePersonalViews, ViewFormPages, Open, ViewPages, CreateSSCSite, BrowseDirectories, BrowseUserInfo, AddDelPrivateWebParts, UpdatePersonalWebParts, UseClientIntegration, UseRemoteAPIs, CreateAlerts, EditMyUserInfo"
$contributePermissionLevel.Update()
$web.Dispose()
$site.Dispose()