Open the image in Single Photo View and click the edit icon.

Access the color options by twirling down the arrow next to Color (on a computer or tablet) or tapping the color options icon (on a phone).

Select Invert to switch the negative to a positive.

Use the AutoColor icon to correct and balance colors, and make any other desired refinements using Mylio Photos' editing tools. You can also select the AutoColor preset from the presets section on a phone.