Installing version 5.0.2

[deadhead18]

Running v 4.4.4 and Nexus is really slowing down. A friend suggested updating operating system and I went to the download of 5.0.2 which keeps appearing on my notifications. Clicked to download it, which it alleges it had done and verified. Clicked on the reboot and install button, countdown starts and then message rebooting and installing appears but nothing else happens. How do I get version to install? Help please. Graham