You can edit assignment hours using the Timeline in both the Project and Resource Planners
Using Rapid Edit
To edit assignment hours per cell:
Select your zoom level (day, week, month, or quarter).
Double-click on a cell in the Timeline to edit. Edited assignment hours are based on the zoom level. For example, if you are in a week zoom level, the hours you edit are applied to the week.
Press Enter to continue or use either the Tab key or right arrow key to move to the right in the row. Use the Shift + Tab keys or the left arrow key to move to the left in the row. You can change edit mode at any time between the Total Hours Constant and End Date Held Constant modes. These correspond to the Hold total assignment hours constant setting in the Planners - Project and Planners - Resource custom settings.
Save your changes.
Stretching Assignment Schedules
You can drag the assignment bar horizontally to make changes to the entire assignment schedule.
Click on the selected assignment bar and move the cursor left or right to decrease or increase the entire assignment schedule.