If you haven't installed all available updates with Windows Update you may not see the upgrade in Store. If so, start the Control Panel and run Windows Update until there are no more updates to install. It may still take some time for the upgrade to appear.
You need a Microsoft account to be able to use the Store (where the upgrade will be displayed in the leftmost position). The account will need to be linked to an email address, and you need to specify your contact details as well as personal details, but you don't need to specify a credit card number.
The update app encourages you to work with other programs while upgrading, but if you return to the Store app, you need to restart the upgrade again to see its progress. But it just continues the upgrade process.
Don't expect the upgrade to work smoothly. There seems to be almost as many crash scenarios as there are PC configurations. Personally, I had to uninstall all antivirus software to avoid complete freezes during the upgrade process. It took me days of failed attempts before concluding this.