There are a couple of options for you.
You can approach your wife, let her know you had the test done, and that you want to discuss it with her. She probably is not going to be happy about it, but that is something you both will have to talk out. You can also add to your discussion that you want to be the one to tell your daughter(s) about it. However, since they are her biological children, she may balk at this. Let her know how strongly you feel about it and see if she will agree.
Another option is telling your daughter first. This option has the most problems for it because it will most likely cause tension between you and your wife. Also, it aligns your daughter with you (at least that is how it will appear to your wife). You also need to consider your daughter's feelings about how she will react to the news. She probably already knows something isn't right but may not be sure. She may have some feelings of betrayal, anger or sadness. She may also feel resentment, especially towards your wife, because she was not told of this.
The main issue is going to be trust. Your wife is with someone else, so there is trust issues there. She lied to you and to your daughters about you being the father of your first child. And now whatever you choose to do with the DNA results you are caught causing conflict and trust issues as well. Your wife is probably going to fee you went behind her back to do the test. Not that it is your fault, it is just the nature of the situation.
I highly recommend that you all go as a family to counseling. You need a neutral person to help you sort this out and make sure everyone can air their feelings about this and resolve how they feel. You can ask your doctor for a referral or if you attend church, your pastor is a great person to either offer you therapy, or refer you out. The local community mental health center can help as well. This situation is complicated and needs to be worked out in some other manner than how it will if you do nothing and let it play out. It can cause years of tension and bad feelings that can affect the kids for a long time.
I hope this helped you,