How do I reprint a letter?

Letters are only downloadable and printable when they're in the "Letters to Print and Prepare" column (this is where they go when you first adopt voters). To get a letter back to that column so you can download and print it, use the left-facing, green arrow button beside the name of the voter to shift that letter from your "Letters You've Prepared" column to your "Letters to Print and Prepare" column, where you can download and print it again. Here is a screen-grab that shows how those buttons work.