@Gimble, @Mdp : Thank you
It's downloaded, I've take the file 'Librera-7.8.34-c518-arm64.apk'. Even when the security is removed for unknown source, the application is not installed.
I've put the file in the 'Download' directory and select it. The message is 'App not installed'.
Any idea ?
P.S. : Sorry my 'newbee' questions, but it's the first Android device which I can use ...