Is there an iPhone app, that help me reduce usage of the smartphone , by change it into an old style feature phone? Maybe I can turn on smartphone mode for 1 hour per day, but besides that the phone can only send text message and make calls.
It would help to understand what exactly are you trying to accomplish?
From what I read I think you want to limit the Smartphone Capability but want to use it for maybe 1 hour a day? So is this a Data restriction question?
You can always turn off the Cellular data and or restrict what Cell data can be used for. This will allow you to answer phone calls but in order to use any of the Data features of the phone you will need WiFi.
To do this go to your Settings and locate the Cellular icon and tap it. You can hit the master switch and turn off all cellular Data or selectively turn off data for each app. I hope this helps.
There will not be an app for this as apps are not allowed to change the system in this way.
The closest you can get to this is iOS Restrictions, which require a passcode to re-enable access to certain features on the device. You can configure which features are restricted on the device in Settings → General → Restrictions. There is no automated schedule for Restrictions — you will be required to enable it and disable it yourself.