Well, the short answer is you can't, unless your tree doesn't include siblings.
How many people are in your tree? And more importantly, how many generations?
At the bottom center, click on "Show Options", then to the right of the colored boxes, choose the number of generations of parents (ancestors) you want to display above the person you've selected, and also choose the number of generations of children (descendents) you want to display. This will get you the person you've selected and their partner(s), and that person's siblings.
In addition you will get all children of the selected person and their direct descendents, up to the number of generations you selected of children. This will include the partner(s) of each of the children UP TO BUT NOT INCLUDING THE LOWEST LEVEL/GENERATION OF CHILDREN. The rule is, if someone's children are displayed, then both parents are displayed; at the lowest level, their children are not displayed, so therefore their partners are not displayed either.
You will also get all DIRECT ANCESTORS, but not siblings of those ancestors, with an exception (see below). For the person you've selected, you will see all their siblings, that person's parents, their grandparents, their great-grandparents, their great-great-grandparents,... up to the level/number of generations you selected for parents.
Finally, and this is important, choose the number of generations of "Others" you want to display. This last one will include siblings and descendents of the PARENTS ONLY of the person you've selected. There is no way to display siblings of grandparents or above, nor those siblings' children (descendents). The higher the number you select for "Others", the more generations of the parents' siblings' descendents will be displayed. Again, you won't get siblings of grandparents or above, nor (of course) any of their children, ONLY those of the parents of the selected person.
Partners of descendents are included if their children are also displayed (an additional level of children), but not partners of ancestors, with the exception of only the parents, described above.
So, if you've been filling your tree with siblings of ancestors, you won't be able to see them unless you select one of those siblings (or their child and use "Others"). This may make a great deal of your tree hidden, but the alternative would likely be wildly unmanagable and unnavigable.
