How to Add a Parent Incident (Major Incident) to Multiple Child Incidents

From the left navigation menu, under Incident, select Assigned to me (or Open)

Filter down to the Incidents that you would like to add to the Parent (Major Incident)

From the hamburger menu    on the upper left hand side of the page and select Personalize List Columns 

Search for Parent and select Parent Incident (NOT Parent), once selected move Parent Incident up in you Personalized Columns List, next to Number, in order to more easily see both at the same time)

Next left click the first cell in the Parent Incident column, it will become outlined in blue

Hold down the shift button, left click, hold and drag your mouse down from the first cell, to the rest of the cells in that column.  All cells will become highlighted in blue

Once highlighted, double click the first cell again.  A Parent Incident search box will appear 

In the Parent Incident search box, type in your Parent Incident (Major Incident) number and then select it from list that appears and click save

Once you click save all incidents will update with the same Parent Incident number