Tap on the 4th icon in the side-bar to switch to the Transformation tool. You then have access to the transformation UI which is a box surrounding the selection with 8 handles and a rotation bar.
If nothing is selected when you switch to the transformation tool, you will transform the existing pixels on the current sheet.
In the middle of the box is the pivot point which you can move anywhere you want.
The top and bottom handles allow you to resize vertically.
The right and left handles allow you to resize horizontally.
The corner handles allow you to resize both vertically and horizontally.
Hold one finger or Shift while you resize to keep the ratio.
Drag the rotation bar to rotate.
Hold one finger or Shift while rotating to snap it every 15° angle.
Move the pivot point located at the center of the box to change the behaviour of the transformation.
Hold one finger on the screen or Shift while you transform to snap the box to the main guidelines of the canvas. It also allows you to move the box while keeping its horizontal or vertical alignment.
The pivot point can also be snapped to the guidelines while using this gesture.
Enables the zoom and pan gestures. If it isn't enabled, you will be able to transform the image with your fingers. When enabled, the transformation can only be made by handling the transformation box and you can zoom and pan with your fingers. When it is disabled (white), you can use the following gestures to transform:
Pinch with two fingers to shrink, and pinch them apart to enlarge and keep the ratio, and rotate by making a rotation movement.
Values are displayed at the top of the canvas during the transformation, tap on them to change them manually. Tap on the values to change them and tap on the px to switch to %. The Lock allows you to keep or change the ratio.
You will find the transformation options in the additional side-bar:
The Quick Transform allows you to switch automatically to the transformation tool after selecting an area. 
Mirrors the content horizontally. 
Mirrors the content vertically. 
Reapplies a previous transformation. It can only be used if at least one transformation was applied before. 
Resets the transformation. You can switch between 2 modes of transformation:
Affine transformation is the basic transformation mode which allows you to change the position and the size of the selected pixels. 
Perspective transformation is the transformation mode that allows you to change the position and the handles of the transformation box independently. 
Defines the quality of the transformation to High or Low. In most cases, the high quality is the best option because it smooths the outlines with different values of opacity, but in some cases, using the low quality improves the quality of the transformation (e.g. pixel art):
You can apply the transformation by tapping on the Transformation icon, which switches back to Selection, or by moving to another frame. If you move to another frame where there is no pixel, it switches to Selection. If you wish to apply the transformation to several sheets and several layers, you need to select the desired sheets in the timeline before using the transformation tool. To select several frames, double tap and drag over the sheets.
When multiple sheets are selected, the transformation box will contain the pixels of all the selected sheets.
