Before you remove speed sensor check battery isn’t on way out can cause fluctuations
If ok remove sensor held by 10mm screw/bolt You may need to slightly twist sensor to remove it
Disconnect plug now Check speedo by using a multimeter set at ac across two terminals in Block if you turn speedo cog by hand it should show a reading (there is a you tube video showing how to do this not sure if it’s your version of Micra) hope this helps