To the left of the + sign up by the timeline, click those bars called global visibility. There is a drop down menu for your ruler, tempo, etc. Click signature. This will add a signature track. With toggle snap on, put your line on the beat where you want to change the time signature. Right click in this signature track area and you will get the option to change the time signature or key signature. Click on the option and you will get a pop-up window. Make your changes and click ok.