Unable to update apps due to old account on iPhone

Correct! You will have to delete those apps and download them again using your new account. Apps are always tied to the Apple Account used to download them, and if you do not have the credentials for the account that downloaded them, they will not be able to be updated.

1. Touch and hold the app on the Home Screen.
2. Tap Remove App, then tap Delete App to delete it.

Then go to the App Store and search for the app. Make sure your App Store is using the correct Apple Account by tapping your user icon in the upper right. You should then be able to "Get" the app again using that account.

Another place you can look to make sure purchases are associated with your new account is at Settings > [name on top] > Media & Purchases. If you see your old Apple Account here, that is the one being linked to your purchases.

You want to make sure you are Deleting the app and not choosing the Offload app option. Once it has been deleted, go to the App Store using your new Apple Account and it will say "Get". Deleting the app does result in any data stored on your iPhone from the app being deleted too.

You can review the Apple Account being used by the App Store by opening the App Store app, then tapping your user icon on the upper right part of the screen.

om45 wrote:

app won’t update because it was loaded when I had an older account

If you don't remember the old credentials then delete and redownload with your new account.