To erase, use the Erase Tool
Click the Erase Tool
Move mouse over the grid.
To erase one stitch, click on the stitch.
To erase multiple stitches, click on the stitch on which you wish to start, keep the left mouse button depressed and move over the stitches you wish to erase. When done, release the mouse button.
You may also erase large areas by selecting the area, and using cut.