Open a Mac app from an unidentified developer
If you try to open an app that isn’t registered with Apple by an identified developer, you get a warning dialogue. This doesn’t necessarily mean that something’s wrong with the app. For example, some apps were written before developer ID registration began. However, the app has not been reviewed, and macOS can’t check whether the app has been modified or broken since it was released.
A common way to distribute malware is to take an app and insert harmful code into it, and then redistribute the infected app. So an app that isn’t registered by an unidentified developer might contain harmful code.
The safest approach is to look for a later version of the app from the Mac App Store or look for an alternative app.
To override your security settings and open the app anyway, follow these steps:
In the Finder
on your Mac, locate the app you want to open.Don’t use Launchpad to do this. Launchpad doesn’t allow you to access the shortcut menu.
Control-click the app icon, then choose Open from the shortcut menu.
Click Open.
The app is saved as an exception to your security settings, and you can open it in the future by double-clicking it just as you can any registered app.
Note: You can also grant an exception for a blocked app by clicking the Open Anyway button in Privacy & Security settings. This button is available for about an hour after you try to open the app.
To change which apps can open on your Mac, choose Apple menu 
in the sidebar, then go to Security on the right. (You may need to scroll down.)
To change your security settings, see Protect your Mac from malware.