Windows 10 Upgrade Assistant is stuck at 99%
We are aware that a set of users are reporting a slow or seemingly-stuck upgrade experience when attempting to upgrade to Windows 10 or update between versions of Windows 10. The upgrade process usually takes 90 minutes or less to complete, but there is a very small subset of PCs, usually older or slower devices, where the upgrade process can take longer than typical. This situation is not related to a specific upgrade tool and has been shown to happen with Windows 10 Upgrade Assistant, Media Creation Tool, USB media, and ISO media.
When Resetting the your windows it’s getting stuck at 99% and it’s also taking much longer time then usual so either start with a fresh and genuine copy of windows or avoid until you have the setup.
For some user stuck at 99 % rebooting might help but only during reset and no upgrade.
The process takes usually 4hrs or longer. According to the offical msdn website support panel.
Start>Settings>Update and Security>Recovery>Reset
But its better to avoid and install a fresh copy because 60 mins of fresh installation is much better than 240 min of resetting though the feature is great for a user without any installation options.