To update your profile picture: iOS - Go to 'My Profile' in the bottom, right hand corner. Once you see your profile, go to 'Edit' in the top, right hand corner of your screen
Android - Go to the "..." in the top, right hand corner > My profile > Go to 'Edit' in the top, right hand corner of the screen
To change your main profile picture:
iOS - Tap on the pencil icon on the biggest image. A menu will pop up to give you options of where you can upload a replacement image. Or, simply tap and drag the image you want to the main profile spot.
Android - Tap on the main profile image. A menu will pop up to give you options of where you can upload a replacement image.

To add a new photo to the rest of your profile:
If your profile does not have 7 photos, you can tap on the ""+"" icon to upload a photo. If you do have 7 photos, all you have to do is tap on the ""X"" to delete a photo you don't want and then replace that image by tapping on the ""+"" icon
You can rearrange your profiles by simply dragging and dropping to the spot you want on your profile, including your main image.
Once you're done, tap the arrow in the top, left hand corner to see a preview of what your profile looks like with the new changes.