Open the Editor and select the measure where the meter should change. The time signature stays in force until another one replaces it, and a song that never sets one plays in 4/4.

 

On iPhone and iPad, tap Symbols at the bottom of the screen, which lists the time signatures. On Android, tap the large button showing the current time signature. On Mac and Windows, choose from the time signature dropdown in the Editor panel.

 

Put a left barline at the first space of the first measure, just after the time signature, so the player reads the start of the song correctly. A left double barline is the usual choice.

 

For the full guide, see Time signatures on the iReal Pro website.