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You cannot.
It's a built in security measure to prevent an unauthorized user from making app store purchases.
 
I kinda like it. Prevents my six y/o nephew from mischievous app installing. 🙂
 
I don't mind my password being required to confirm any purchase on the phone, but I would like a way to skip the password for updates.
 
It's all tied into the UNIX underpinnings of iOS. There's no way around the password prompt when using the App Store for updates or installs.
 
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