If string holding together the hemming on a pair of pants that is being worn has come out during the nine days, is it permissible to fix it (no changes are being made to the hemming, just ascertaining the pants don’t get ruined/look shlumpy)?
Small sewing repairs are not included in the issur and are permitted. Therefore it is allowed.
Moed L’chol Chai 18, Kaf Hachaim 551-107, Rivivos Efrayim 5-376 (11), Orchos Rabeinu 2- pg. 129, Ohr L’etzion 26-2.