By editing the details, to turn your grandfather's record into your grandmother's.
You should see a wrench icon (spanner) above the photo on the right. Click on that, and you'll see an Edit Record menu option. Click on that, change the fields you want to change, and then click on the tick symbol on the top right to save it.
Be aware that you may have trouble entering or changing some fields such as Age at Death. There are a number of bugs in the new version of BG which have not been fixed yet.
But I have had no trouble changing the name and date fields, which I presume are the main details you are wanting to change.