Perhaps the hardest thing for beginning editors to feel confident about is when to make the change, and when to query the writer about making the change. Even experienced editors can lose sight of this distinction.
In general, your approach will have to do with the levels of edit.
In other words, always be aware of the reason for changing something. As editors, we need to guard against unnecessarily changing the writer’s voice or the content of the writing.
To sum up —
Make no change that you could not defend if asked to.
Make the most economical change that solves the problem you’ve identified.
Note that sometimes what’s needed is a heavy edit, and when this is what’s required, this is the most economical change. Note too that in some environments, editors take the lead — as might be needed — in shaping the content to suit the situation and the audience. That’s something of a different situation. But even in this case, make it a practice to always have a reason for the change.
Developing an editing process