I want to use FMRTE to change the stadium. But in FM2017 when I change the seat numbers and click save, it change back to the default number. 

I used FMRTE to change stadium in FM2013, but I want to know how to change it in FM2017?

I am sure you are running app as suggested....Run as administrator- to do this right click on the app and select RUN AS ADMINISTRATOR

if you are then follow this as I have just checked no issues changing stadium capacity  in FMRTE for me

 

Yes, you are right. The virus tools hold up the modification. 

But I found another problem, when I change the seat numbers to more than 125,000, the game will reset it back to 125,000. 

So, 125,000 is the maximum capability of a stadium? Is there any way to change it  to a larger number such us 200,000?

Thanks. 

that could be the max

check in pre game editor too there is a max setting there 

I havent got access to FMRTE/game now