When opening a new file, the first measure defaults in 4/4 time. To change the time signature, select the time signature in the staff. This will open the Time Signature Palette, where commonly used time signatures will be displayed. For an uncommon or custom time signature, select the "..." button and a new window will open.
Use the up/down arrows to select which time signature you would like to apply, then press "Okay" when done.