I strongly advise that your phone is plugged into the power and if necessary, forwarded to another number while the upgrade is happening.
This upgrade took some time on my phone once I found out that it could be done.
I may have missed something but I waited a long time for this, the only reason I found it was due to the current lack of new windows phones on the market.
Go to the windows app store on your phone and search for Upgrade Advisor download and install.
There are a couple of things to do once updated, on checking my storage after the upgrade, it was 4.44 GB used out of 7.28 GB main memory and small amount used on my SD card.
Things look pretty much the same, all in all, good upgrade.