http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.sppermission.aspx

SPSite oSiteCollection = SPContext.Current.Site;
using(SPWeb oWebsite = oSiteCollection.AllWebs["Site_Name"])
{
    SPList oList = oWebsite.Lists["List_Name"];
    SPPermissionCollection collPermissions = oList.Permissions;
    SPUserCollection collUsers = oWebsite.Users;

    SPMember oMember = collUsers["User_Name"];

    oList.Permissions[oMember].PermissionMask =
        SPRights.AddListItems | SPRights.EditListItems;
}