Windows will warn you to save your files because the PC will restart; click Start upgrade when you're ready. You'll see a progress bar on-screen – you can carry on using your PC but it's going to restart without warning, so you don't want to be in the middle of editing files when that happens.
The upgrade will take about ten minutes and then reboot, which takes another few minutes, again with a progress bar, before you're back at the login screen. When you log in, you'll see the confirmation that the upgrade has completed; this is another UAC notification and you won't be able to do anything on your PC until you close it.
New Pro features
Your existing Windows account will be there, complete with files and installed applications, but you'll find the extra Windows 10 Pro features available. For example, you can defer updates in Windows Update, turn on BitLocker, use the Group Policy Editor to change settings and run Hyper-V for virtualisation.
Depending on what your company has set up, you may be able to connect to your business network using Direct Access (instead of a VPN) with Windows 10 Pro. If you use Azure AD to connect to services, you'll get single sign-on to cloud apps that your company subscribes to – and if they've set up their own Store for apps you can use, that will be available. You can also set a Windows 10 Pro PC to run only one application, as a kiosk, if you want; that's called Assigned Access.
There are other Windows 10 Pro features that companies will be able to turn on when they arrive later in the year, like Enterprise Data Protection, which puts every business file in its own encrypted container. Those will need management tools like Intune or System Center Data Protection. If it's your own files you want to protect, turn on BitLocker to encrypt the entire drive.