In Power BI, you can modify existing groups as much as you like.
To edit any group, right-click on its field either on the ‘Legend’ bucket or on the Fields list (both works).
Then, select ‘Edit groups’ from the options.
You’ll then see another box open up.
Let’s add ‘Oct’ to the list.
All you have to do is click the value first, then the group where you would like to add the value.
In this case, click ‘Apr & Jan & Jul’.
Then, click the ‘Group’ button below.
After that, the value will be added to the group.