No, this prompt does not involve The Box Tops' 1967 song, "The Letter" (but if you can work it in, go for it!).
Rather, write a letter to your younger self. This type of poem is called an epistolary. The tone of the poem can be whatever you wish. The form can be free verse, a prose poem form, or a traditional verse form.
You can write a letter of advice, comfort, or a look forward. You can focus on one meaningful event (parents' divorce, first heartbreak, etc.). You can take an event and compare and contrast your feelings from the point of view of yourself as an adult and as a child when the event took place.
The Academy of American Poets has some examples of epistolary poems here. You can also find more information from the New York Public Library here and in an essay by the poet Paisley Rekdal for Assay.