Open Internet Download Manager. From the top menu go to Help > Check for updates... or Help > Quick update for older versions. Then you should restart your computer as the software won't update without restart. If you have the latest version of IDM you should see this:
Otherwise you will a dialog box that suggests you to update IDM. Click Update now to install the new version.
If for some reason IDM fails to connect and update please consult IDM's troubleshooting instructions.