Inserting notes

Hi everyone. How do you go about placing say a "semi breve" or a "minim" onto the stave when you have already placed crotchets in a bar? At present all I can do is insert one of the other. If I already have a semi breve or minim and then want to insert crotchets or quavers as well in the same bar, the semi breve changes to either a crotchet or quaver. Can you help?

Thanks, Barry