The most basic: Tap the "FX" icon (upper right) > tap the "Basic" group > Sharpen.
If you want to specifically affect a portion of the image: On the set of tools on the left, tap, hold and swipe down to the Paint Tool, keep holding and swipe on over to the Effects Paint tool. Tap the currently active effect to change it to "Sharpen." Adjust the intensity. Tap the checkmark. Start painting. (You can adjust the brush size on the left.)