Hi,i cant update my instagram app , so it say to need payment

You can download free apps, but you may need to have a valid payment method on file, even if the app you want to download is free. 

See -> If you can’t download or update apps on your iPhone or iPad

If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

scroll to point 2 & 3  ⤵️

Add a payment method

You may need to have a valid payment method on file, even if the app you want to download is free.

Add a payment method

• If you want to change your card, add first the new card, then remove the old one.
• or if you use your Apple ID balance, set your Apple ID balance as your default payment.

Resolve a problem with your payment method

If your payment method has expired, declined or failed, or a message says "There is a Billing Problem with a Previous Purchase"

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service 

Many free apps still require a payment method on file because they offer in-app purchases. If you do not have a payment method added to your Apple ID, please follow the directions in the link above your post.