How to change "preferred" Parents from the adopted parents to biological parents

I cannot figure out how to change the preferred parents from the adopted parents to the biological parents.
I have a great grandmother that I am trying to research, however; the information in my tree is based on the information from her adopted parents, not her birth parents. (I actually have 2 great grandmother's like this) ugh!... I am trying to show the DNA line rather than the who raised them line. At one point, I wanted to delete all the people from the adopted parents line, but I was sort of chastised for doing that. I do not understand what the problem would be because the current information (as adopted parents) is for those persons attempting to do "True" DNA genetic genealogy. What do I Do? Can someone please advise as to how I can do accomplish this. It drives me crazy and I want to delete my tree altogether. :(
Answers
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You do it from the child page. I have an adopted relative whose lineage I am doing and this relative has 3, not 2 sets of parents. Birth mom and birth dad never married but married other people.
Do not delete any true relationship from a person. If a child stayed with a couple for even one year that information should be on the tree with sources (if possible) and explanations.
When you go to the person page for the adopted child, you can see an on/off switch to show all parents. Near it is the option to set preferred parents. You can switch this as often as you like. In the case of my adopted relative, I change the preferred couple every now and then so that I can see the pedigree view of who I'm working on, either birth mom or birth dad.
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