Welcome to iscore and the forum
As a user, I encourage new users to watch the online training videos and score a practice game or two..before going out in the "wild"
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Yes you can make some non game altering edits after the fact using the pitch by pitch editor, (game manager> select game> pitch by pitch > select activity to edit... Edits not allowed will be unavailable or grayed out)
You can also always go back using undo and then re score from the point of your mistakes.. ( depending on when mistake happens can be easy or time-consuming)
Some Examples of non game altering edits which you can make at any time through pitch by pitch editor...are substitutions, change hit to error, or why out. But you can not change an out to safe. That alters downstream events. Baseball has a number of interdependencies that make free form editing a challenge. There are many posts on this board that explain in more detail this topic.
As it sounds like you have not tried pitch by editor yet, I suggest you play with that for a while and see if it helps solve your immediate needs.. You can also manually override stats if that suits your needs better.