The install instructions are the same if you have purchased the DVD version or the USB version.
I do recommend to copy all the files from the USB / DVD’s to your PC as this reduces the chances of transfer errors especially with the DVD’s.
You will see four folders on the USB Disc 1,2,3,4 on the Discs each DVD is labelled with Disc one Disc two etc. So if you copy them all to your PC you will end up with the below folders.

It is important you install the folder in order. So disc 1 then 2 etc if you don’t not all the updates will be applied. So go into each folder and select just the PC icon and double click it as shown below, ignore all the other files. DO NOT CHANGE THE INSTALL FOLDER.

Skin Issue
Please note that if you bought or downloaded the update before the 23rd November there is an issue with the Skins, this is due to SI changing the pannels in the default skin.
First delete the existing skins. So go to documents/sports interactive/Football Manager 2020/skins within there delete all subfolders. As shown below

Now install the new skins pack from the below link
In game settings
There are a couple of in game settings to change. So when you load the game up select preferences then in the search box search for the word skin. Then change the settings so they match the below.

Then to apply the game you just need to change the skin in order to be able to view the updates.

Real Name Fix
To get real names working you need to do the following folder
Program Files (x86)\Steam\steamapps\common\Football Manager 2020\data\database\db\2010\lnc\all
Within that folder delete
- fake,inc
- lic_dan_swe_fra.lnc
- nleague.lnc
Now go to the following directory
Program Files (x86)\Steam\steamapps\common\Football Manager 2020\data\database\db\2010\edt\permanent
Delete the fake.edt file
Now go to the following directory
Program Files (x86)\Steam\steamapps\common\Football Manager 2020\data\database\db\2010\dbc
Delete the following files
- Zebra Award.dbc
- Zebra Turin Fake Staff.dbc
- Zebra Turin new.dbc
Now go to the following folder
Program Files (x86)\Steam\steamapps\common\Football Manager 2020\data\database\db\2010\dbc\permanent
Now delete the following files
- brazil_kits.dbc
- forbidden names.dbc
- Licensing2.dbc
- Licensing2_chn.dbc
Now apply the real name fix from the USB / DVD / Download
D:\Program Files (x86)\Steam\steamapps\common\Football Manager 2020\data\database\db\2010\dbc\permanent