To view completed milestones and actions on a given initiative, start by going to the detail page of the initiative.
Scroll down to the second section “Execution plan” and click the FILTERS option placed below the timeline on the right;
Once clicked, turn ON the “Show completed” function and unfold the dropdown. Proceed by choosing your time range and finalise by saving your filter;
If you wish to turn off the completed items once again, simply return to the FILTERS in your “Execution plan”, click OFF in the “Show completed” options.
The same should be followed using the FILTERS on “My overview”.