Q: My phone is asking for credit card details even for updating free apps.what to do

Update the credit card details. Apple uses your credit card to help identify you, and that is part of your Apple ID. Even though the apps are free, many have in-app purchases, and making any purchase in iTunes, Apple makes sure it is you. All you have to do is update the account. If you no longer have the account, then you will need to take a look at this about trying to select None for payment method if you have an established account. Why can’t I select None when I edit my Apple ID payment information? - Apple Support

If not, then you are going to have to either purchase an iTunes gift card to establish your account if you have already created the Apple ID, or you can create an Apple ID without a credit card by looking at this support document. Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…