The antonyms for the word "sewing up" can be "undoing," "untangling," or "loosening." These words signify a process of loosening the grip, untangling the threads or stitching, which were initially sewn up or put together. "Undoing" is a term used when referring to the opening of a seam or an already sewn-up area. "Untangling" is more appropriate when used to refer to reversing the process of stitching or sewing up material. "Loosening" refers to a general process of making something less tight or secure, which could apply to sewing up as well. In summary, if the reference is to the act of undoing, untangling or loosening, antonyms for sewing up could be applied.