The easy way is to long press on the gallery app icon and tap the circled "i" on the upper right of the menu. This will take you to the configuration settings. Once the settings open, scroll down to the storage section, tap it and then select "delete cache". The long way around to the same spot would be to open the settings app, go to apps, scroll to or search for the gallery app, and continue as above.
It is supposed to "overwrite" the existing photo -in a way, but, what it actually does is saves the edited copy as the one that shows up in the gallery and hides the original in case you want to revert to the original. So, what is actually *should* be doing is replacing the original in the gallery with the edited version and that edited version is what should be showing if you put it in another app.
If you are sure you never want to revert to the original, you can just save the edited version and delete the original(s) or at some point clean out the hidden originals. There's an option for that in the My Files app. The invisible originals do take up space, so if you do a lot of edits, you should regularly be clearing them out. Whichever way is easiest for you.
