Go to the song you would like to edit, tap/click on the Edit icon.
Tap/click on the measure where you'd like to change time signature.
iPhone/iPad: At the bottom of the screen, tap on Symbols and it will show all the available Symbols including Time Signatures
macOS: in the Editor panel choose the desired time signature from the drop down menu in the editor panel

Android: tap the big button indicating the current time signature – i.e 4/4