correct way to unlock an engine 'private' file and make it editable. -crash alert-

I unlocked from VS 2019 the ‘ProjectileMovementComponent.cpp’ and overwritten it with a very small change to the
UProjectileMovementComponent::HandleImpact method
On build the UE editor crashed.
What would be the correct way to do this? I thought any project files were in kind of ‘user space’ buit this points to the main UE install folder.
Thank you!

You could give us the error and log so we could find the source of issue.
If you compile editor from source, you can modify any file.
Also, you can copy ProjectileMovementComponent.cpp into your project with your changes and use it so that you won’t have to recompile whole editor

I submitted the crash report, so hopefully that will help, I tried again to restart everything, no more crashes, but the change I made is not reflected in the game (similar to a post about changing for example InitialLifeSpan of a projectile, nothing changes in the game InitialLifeSpan not working - UE4 AnswerHub )

You download the source code from github. Then you can change the engine code and finally build it.

unless i misunderstood you, I’ve never seen or heard anything related to modifying binary builds from the epic games launcher.