The easiest way for ME to transpose a song is to use a method I learned when studying accordion, and I was responsible for transposing many symphonic instrument parts - such as a 'Bb' Clarinet part - to 'C' so it could be read and played by an accordionist ...

Take the key the song is written in - in this case Db - and write out the Db major scale -D♭ E♭ F G♭ A♭ B♭ C ... then beneath that write out the scale for whatever key YOU want to play or sing the song in ... let's say the key of 'G' - G A B C D E F#... lining up the notes of the 'G' scale directly below the notes of the Db scale so that the G is directly below the Db, the A is directly below the Eb, etc. etc. ... you now know what chord in the key of 'G' corresponds to the chord shown in the key of 'Db' ...
I know that well schooled and knowledgeable musicians here would be able to transpose more easily using the 'numeric' method, but as I said, this is the easiest way for ME ...
Good luck with the song ...