You will need to select your image while in Edit Mode to make it the active document element. You will know it is the active document element because of the red border surrounding the image:
Then click on the Insert Module. You will see the options for Clip and Crop. They will only be accessible if your image has been selected.
Select the Clip option. The icon for the mouse will change into a cross. The red border will also disappear. Hold down the left mouse and drag the cursor to highlight the area of the image you would like to keep.
When you release the mouse button you will see a preview of what the final result will look like.
You can reposition the area you would like to keep. Hold your mouse button down over the selected and move the cursor to change the position. Using the circles on the border of the selected area you can change the size.
Click on Clip again to finish the action. You can select the image and click on Clip again to reposition the clipped area at any time. Soda PDF will remember what the original image looks like.
Choose the Crop option. The mouse icon will turn a cross. The border around the image will turn black.
Hold down the left mouse button and move the image. When only the part of the image you wish to keep is visible, release the mouse button:
When you release the mouse button you will see a preview of what the final result will look like. You can reposition or resize the cropped area. Once you are finished click on Crop again to continue. The selected area will be all that remains.