To shorten correctly, you would have to unbraid the strands down to the first loop, which means removing the serving, and then rebraid. Some longbows require a smaller loop in order for the string to stay on; others will work with a recurve sized loop. If yours requires a smaller loop, then you would need to unbraid the whole string. I suppose it would be possible to re-use the serving, but it will be a twisted mess when you take it off. Lot of effort, particularly if you don't know how to make strings. Probably better to buy a new string and put the old one on an “orphan string” nail, in case something turns up later that it will fit.