I edit a Mod's .mas file, but it updates on steam on launch and reinstalls the original files

[formulabspec2014]

How do I stop this?

 

[Coutie]

How do I stop this?

You should not do that, and instead make an update of the mod. This is for liveries, but it's pretty much the same.
https://docs.studio-397.com/users-guide/custom-liveries-how-to-make-an-update-rfcmp-for-league-use

 

[Manfredk2]

copy the mas to another place, edit it and then install it as update with the mas tool

edit: Coutie was faster