Fixing a Sefer Torah on Hol Hammo'ed is forbidden if it requires a Sofer (scribe) to repair it by adding a letter (as opposed to simply separating letters or repairing it in a way that does not require a professional scribe). This assumes that the Synagogue or Minyan has other Sifrei Torah (Sefer Torahs). If not, it may be repaired on Hol Hammo'ed even by a professional scribe.
If the Synagogue only has one Sefer Torah, but another one in the vicinity has several, we do not require them to bring one from the other congregation. Indeed, there are complications with doing this according to the Zohar. As such, they may repair their Torah.
A Synagogue that has exactly three Sifrei Torah, and finds an error in one of them on Hol Hammo'ed of Sukkoth, is permitted to fix it, even though the other two are usable. The reason being that on Simhath Torah we take out three Sifrei Torah. Even though one could make do with two by rolling one of them, this is not the preferred method since it puts a burden on the congregation who have to wait. It is also more honor to the Torah to have three Sirei Torah and gives more pleasure to the three Hathanim who are called up to the Torah on Simhath Torah, who will each have a Torah to read from.
(See Sh. A. 544:1. Kaf Hahayim ibid: 3, 6, 14)
