To add an Estimated Completion Date or ECD to a single project you will need to be in edit mode of the project you want to update. Once you are in Edit mode in the Project Dates Section there is the Est Completion Date field, change this to your desired date and then click the blue Save button on the Project Screen.
To Edit multiple ECD's on multiple projects you can go under Active Project portlet or Active Project Custom Search, make sure you are in Edit mode on your screen.
Select your first cell you want to change in the ECD column/ result, scroll down to the bottom of what cell you want to edit through. Press and hold you shift key on your keyboard and then click the bottom cell with your mouse. This will create a dotted line box of all projects that will update the ECD field.
In your original cell change it to your desired date and then press the Tab key on your keyboard. This will mass edit the ECD fields all at once. If the cells you want to change are not all in the line the same function can be completed with the same steps but using your Control key instead.