Keep on pressing till the update is done, dont leave it.
Better to drag the file in Updater before starting preboot mode,so when Updater sees it you can click it easily without any problem.
But read this thread for how to do.
[Only registered and activated users can see links. ]
I dont know how is in your Nuvi model,but with mine for example I shut nuvi down ,then attach it USB , start it pressing and holding , as soon as I see it on Updater I click OK