You can change profile pic in the app. On the main FB screen, swipe right or tap the 3 bars in the top left corner, tap on your name to bring up your own profile. Tap on your profile picture (or go into your photo album to if you want a new one) to open up the picture. Tap the 3 dots at the bottom to bring up a menu, and "make profile picture" is in there. Just drag the box around and tap on the tick to confirm it. *you can shrink/grow the box by pinch-zooming.