enter %AppData%\Microsoft Flight Simulator into the Explorer.
note : that is a shortcut to C:\Users\XXXXXXyourUserNameXXXX\AppData\Roaming\Microsoft Flight Simulator
In this folder you can delete:
folder cache
folder DCE
folder SceneryCache
folder SceneryIndexes
file ROLLINGCACHE.CCC
EDIT: fixed a typo %AppData… → %AppData%