To manually adjust shutter and ISO, please activate a manual mode (M or SI) in the control panel (menu button bottom right). It's the option on the middle row far left.
Tapping this option will allow you to select A (fully automatic), SI (shutter & ISO priority) or M (fully manual). SI mode is semi-automatic in that you can set shutter speed manually, and ISO will be adjusted automatically, or vice versa.
In order to select actual ISO and exposure time values, tap on either the ISO or exposure time value in the top bar, and the corresponding dials will open.
In shutter & ISO priority mode, you only specify one value, the other will be adjusted automatically and you can brighten/darken the photo with the EV dial.
Note: When you have A-Mode enabled, tapping on either the ISO or exposure time value in the top bar will only lock the current exposure and not open the dials for manually setting values. Locked values are indicated by a white outline around the value label. And a white background means the corresponding dial is shown, and manual control is active.
If your iPhone supports Smart HDR, you might want to disable it in order to get exactly the exposure values you have selected manually.
For more detail on manual controls and Smart HDR, please read 'How do I make sure manual ISO and Exposure are exactly as I wish?'
More detail on our shutter & ISO priority mode can be found here in our blog post.