Open the Photos app
Select the picture
Tap Edit
Tap the Crop/Rotate tool
Tap the Flip icon if available
If you want to invert colors, open Settings
Tap Accessibility
Tap Display & Text Size
Turn on Classic Invert or Smart Invert
If you want to invert a photo with an app, open a photo editor from the App Store
Use the app’s invert or negative filter
Save the edited picture