This can be a bit tricky.I'm assuming that the problem exists in a single line, rather than across your entire range of ancestors.
First and foremost, back up your .paf file and once you start the process of inserting a generation, do not back up the file again until all the changes are made and you're satisfied with the results.I'd recommend working with a copy of the .paf file rather than the existing file, e.g., Smith2.paf rather than Smith.paf
Let's say that you've entered your father as a child of his grandfather rather than his actual father, so the generation missing is his parents.Go to the family group where he's listed as a child and click on his name to highlight him.Switch to the "Individual" tab display (which I think of as the index) and note the RIN for your father.This is so you can quickly find him again if youneed to.
Go back to the "Family" tab display.Your father should still be highlighted and displayed as a child of his grandfather.
Go to the edit menu and click on "Unlink Individual".You'll get all sorts of warning messages, but just unlink him.He's now still in your database, but not linked to any parents.
Go through the process of entering his proper parents so that he's now the child of the right couple.If the proper parents do not currently exist in your database, you need to add new individuals.If the proper couple is there, then you link to existing individuals.If you're linking to an existing couple, it helps to know their RIN's, which simplifies the linking process.
If you've added new people as the parents, you'll then need to note their RIN's and go to the couples that they're the children of and then go through the Add Child process, linking to an existing individual, since they've just been added.
Don't be disheartened if the process does not work correctly the first time, it's tricky and may take more than one attempt.If you need to try again and you're working with a copy, just delete it and start over with your original database making a copy once again.If you've been working on your original database, just restore it from the backup and start over.Remember not to overwrite the backup of your original database until the process is complete.
In summary, the process is to unlink from the wrong parents, add the correct parents if they're not in your database or link to existing people who are the correct parents.If you added new people as the parents, then they, in turn, need to be correctly linked to their parents.