Images and files that you've uploaded to any content can be found and replaced. 
From the Control Panel:
1. Select Site Content
2. Select Browse
3. Select Files

To replace a file:
Search for the file you want to replace.

Click Edit for the file you need to replace.

Click the Replace File button and choose the replacement file. 
Warning: replacing the existing file cannot be undone.

4. When the replacement is a success, you will see a confirmation message below the Replace File button.

5. Save. Upon saving you will be brought back to the main files page.