The purpose of editing the activity duration during Masterplan Pre-configuration phase is to set the Base Duration.

Click on the Expected Duration input field provided in the edit option and increase or decrease the value.
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Increasing the duration will extend the activity end date. In pre-config phase, any extension of activity duration will not be considered as a "delay" and the user is free to re-plan as required. Successors of the extended activity will be re-scheduled to follow the new end date. Similarly, decreasing the duration will advance the end date of the activity.

In the above image, you can see that Float is shown as 0.  Float is the amount of time an activity, network path, or project can be delayed from the start without changing the end date of the project. If Float is 2, it means that the activity start date (or duration) can be delayed by 2 days without affecting the project end date. Activities with Float 0 indicate that any delay caused to them, be it delay to start date or extension of their duration, will end up delaying the project end date as well.

To edit the Activity duration without going into the detailed view, see the video below.
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