While peaks are displayed in a spectrum/chromatogram, select [Processing] in the menu bar and then select [Peaks] > [Edit Peaks] from the drop down menu.
The peak areas turn yellow.
You can also turn the peak areas yellow by clicking the
icon in the toolbar.
When the mouse pointer appears as
, left-click on the dot displayed at the end of the range you want to change and drag.
Release the left mouse button.
The peak area is changed.