It’s pretty basic, just login to new system with your same account.
https://flightsimulator.zendesk.com/hc/en-us/articles/7059727103260-How-to-move-Microsoft-Flight-Simulator-to-a-new-PC-#:~:text=To%20install%20your%20game%20on,your%20game%20from%20your%20library.&text=If%20your%20game%20is%20up,installed%20on%20your%20old%20system.