A hard transposition rewrites the chords in the chart. Use it when the sheet music you are copying from is printed in a different key.
Open the Editor (pencil icon). The key signature sits at the bottom of the Editor panel. Select the key and tonality you want.
Set and Transpose moves every chord in the chart into the key you chose and sets the key signature to match. Set changes the key signature and leaves every chord exactly as you typed it.
Saving after either button clears the song's soft transposition, in your library and in every playlist that holds it. There is no Revert, so duplicate a song first if you want the original to survive.
If the key and the chords disagree before you start, see Troubleshooting - Key signature doesn't match the chords shown.
For the full guide, see Transpose a song on the iReal Pro website.