Problems installing and activating Sierra

My iMac has been running OS X Lion 10.7.5 (for reasons I can't explain here I failed to update it and then was unable to install other updates). I was advised by Apple support that I can install an older OS which would still be better and more recent than Lion. So I went onto this page where it advises the steps to take to install an olderOS, and following the advice here I installed Sierra: How to get old versions of macOS – Apple Support (UK)

Everything went ok and it said 'installation successful', but what do I do now.to actually get Sierra running on my computer? I tried restarting and shutting down and starting up again, but there is no indication that Sierra has been installed; the Mac info still shows 0SX Lion10.7.5; I cannot see where Sierra has been installed nor how to activate it or get it to actually function / overwrite Lion. Any advice gratefully received!