When it's not simply referred to by its number, this poem takes the first line of verse as its title. Why? It wasn't part of the convention of the late fourteenth century to give short poems—or even in some cases, long ones—unique titles. This has partly to do with the reality of manuscript production: you wouldn't take up extra space on a page to place a unique title since it was expensive and time-consuming to procure the materials. Take a look at the manuscripts in the Petrarchive to see the set-up of the pages (and the lack of titles therein).
Canzone(or "Song") 126 is part of a larger work that has several names. Petrarch himself called it Rerum vulgarium fragmenta, or Fragments of Common Things. That "vulgar" bit in the Latin title isn't Petrarch being ridiculously humble. He's saying that the work is written in Italian, or the "vulgar tongue."
The work has also been called the Rime Sparse (Scattered Rhymes), which refers to a phrase in the first poem in the collection. And finally, the title Canzoniere (Book of Songs) has gained popularity in the last few decades. All options are equally correct, so find the one you like and stick to it.