Officially (going by the indicia at the bottom of the first inside page of the comic), the title changed to Uncanny X-Men with issue #142; prior to that, the title was just X-Men with no adjective. However, "Uncanny" had been used on the front cover above the X-Men logo for some time. (Still earlier, "All New, All Different!" had been used, to distinguish the New X-Men from the older incarnation prior to issue #94.) Once "Uncanny" became part of the official title, that freed Marvel to create yet another X-Men title without the adjective, which they did with great fanfare during the "Gimmick Age" of the early '90s (multiple variant covers and all). Use of adjectives in the titles of comics is a long-standing Marvel tradition (Amazing Spider-Man, Incredible Hulk, Fantastic Four), but publishing different titles simultaneously with and without the adjective is a 1990s gimmick.
Some price guides, checklists, and other comics references split the title at the point of the official change, with earlier issues under "X" and later ones under "U"; this results in the famous and influential 2-part continued story "Days of Future Past" (in issues #141 and #142) getting split between the two listing sections.