Unable to upgrade Reminders

I have been kept old reminder style on my iPhone due unable to upgrade on my old iPad and Mac, due old firmware.

Now that I bought a new iPad I decided to go for the upgrade.

However, after selecting Update, the syncing logo starts with no ends. Everything becomes super slow, and after a few minutes I am prompted again to update… what is the issue?? Any help please? It should be so easy but practically is very complicated and does not work.. thanks