If you wish to replace your "father of record" with your "biological father", you can just edit the information of your "father of record", replacing his information with the information of your biological father, but I suggest that you consider this:
Any person in a PAF database can have multiple sets of parents. The parents can be shown with a marriage date or not, and you can use "Not Married" as the marriage date if you wish.
In you situation, I would start by changing your existing parent relationship to Adopted. To do this: 1. click on yourself 2. click Edit > Parents > Relationship to parents > Adopted > Save
4. add your biological father as another spouse of your mother. 5. add yourself as a child in that relationship, but do not create a new record for yourself - instead choose "Select existing individual" and choose yourself by RIN (normally #1) or by using the Indiovidual list. 6. click Edit > Parents > select the biological parents > Relationship to parents > Biological > Save
PAF cannot split the relationship between the two parents. (i.e. Biological to one parent but Adopted or Guardian to the other. That can be explained in the notes.)
After these changes you will be shown as one person with two sets of parents. One relationship will be shown as Biological and the other will not. You can change which set of parents is automatically shown by Edit > Parents > Make Primary > Save.