Change to a different color floss? Certainly. Mark the area. Then select the color you want to use from the Available Colors palette and add it to the floss Palette. DRAG the new color over the color you want to replace and release the mouse. You'll get a little menu for "Replace" and cancel. If you have marked an area, you will have an "arrow" beside "Replace" that had options for entire pattern or selected area. Click on "Selected Area" and the new color (and symbol) will replace every stitch of the old color within the selected area but no where else. You now have one more color.
If you want to replace within an area with a color already selected, then you don't need to add a color first.
Choose the cursor of the arrow not a stitch. move to one corner of where you wish to select, drag to the opposite corner (don't let up on the mouse button), and then release the mouse button. you will see a "box" showing your selected or marked area.