Most phones nowadays have a built-in photo editor (usually either a pencil icon when viewing a picture or an 'edit' option in the menu, also when opening a picture). If that doesn't work, you have a myriad of app options like Photoshop Express, Snapseed, etc.
That being said, if this is a 'true' passport-like photo, remember there are several rules you have to follow for those, including lighting, pose, visible features, framing, etc. so make sure that selfie will work, even when reduced to a 2x2 frame. If you print online on major photo printing offices like FedEx, Walgreens, etc., they usually have digital tools available to both adjust your uploaded picture to the right size and AI tools to analyze the picture to make sure it meets passport criteria.