How to modify the permissions for a particular permission level in SharePoint

$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()