The Relationship button is on the button row of the person page. Other buttons are Facts, Timeline, Relationships, + add a fact, x delete a fact.
Note that before a person shows up as a parent or a child of another person, they must have first be attached as a parent-child One place to do that is at Menu > Persons > Attach/Detach person). Or, if the other person is not in your file yet, you can Add a Father or Child in either the Tree tab or Person tab (in the mini-tree).
A word of caution. The natural (ie blood) child - parent relationship should be shown as the PREFERRED relationship, in order to show the BLOOD ancestors of a person in ancestor reports.
There is no way in FTM to EXCLUDE various kinds of children and their descendants in DESCENDANT reports. So, it is normally not advisable to include step-children as children of step-parents in FTM. Including descendants of an adopted person is a problem in FTM,as those descendants will appear in Descendant reports of an adoptive ancestor, regardless of whether they are shown as preferred or not.