If you require a change to an existing pending activity you will use F10 to open a prompt for that activity. The activity must be highlighted meaning that if it is not already the highlighted activity on the screen you will need to move your cursor to the activity bar you want changed and press enter and F10. The prompt screen is the screen used to initially create the activity. Change whatever data values you like and press Enter to complete the change process.
You may delete or "undo" an activity only if it is the original version. You cannot undo an activity that has an activity note attached to it. Undoing an activity can only be done by the author of the activity within a day of the original creation. After that time you will need to complete (F15) or drop (F10 & F16) the activity and it will remain as history on the contact record. To undo an activity: Use F10 (change activity) and F23 Undo activity with another F23 to confirm deletion.