Open the Photos app
Select the picture
Tap Edit
Tap the Markup icon if needed
Use the Remove tool if available in your app
If your iPhone has Clean Up in Photos, select Clean Up
Brush over the object you want removed
Tap Done
Save the edited photo
If Clean Up is not available, use a third-party app like Snapseed, TouchRetouch, or Adobe Photoshop Express
Open the app
Import the photo
Use the healing or object removal tool
Save the edited image to Photos