var selectedIds = uidoc.Selection.GetElementIds();
using (Transaction tx = new Transaction(doc, "修改参数"))
{
    tx.Start();
    foreach (ElementId id in selectedIds)
    {
        Element elem = doc.GetElement(id);
        Parameter param = elem.LookupParameter("参数名称"); // 替换实际参数名
                                                        // 或者使用内置参数：elem.get_Parameter(BuiltInParameter.参数枚举)

        if (param != null && !param.IsReadOnly)
        {
            // 根据参数类型设置值
            if (param.StorageType == StorageType.String)
                param.Set("新值");
            else if (param.StorageType == StorageType.Double)
                param.Set(123.45);
        }
    }
    tx.Commit();
}
uidoc.RefreshActiveView();  // 刷新界面显示新值