SHARE and then REINSTALL ISSUE

HI,
My son tried to show me how we could “Share” FS2020 on his new super computer. He downloaded it but now I cant seem to get it back . All files are there in my dedicated D FS drive folder but it wants to re download all 106gig.He has since purchased his own version.
To make matters worse i throttled back the band width usage to 20Mbs in the game settings to stop the screaming when i played FS2020 and dragged everything else to a standstill.
It is still using this bandwidth throttle. I cant change it as this is classed as 1 huge UPDATE.

Can anyone suggest an easier method of getting STEAM to reinstall FS2020 from the files that still exist on my PC rather than re download the whole game.?

Grateful for any help.