How I can minimize approval time of my iOS apps/updates in iTunes connect? In smaller updates it take minimum a week in "Waiting for Review" state. Is there any procedure to reduce this time?

You Can Request for Expedited App Review

Here is what apple mentioned for avail Expedited App Review

Expedited App Review

If you face extenuating circumstances, you can request the review of your app to be expedited. These circumstances include fixing a critical bug in your app on the App Store or releasing your app to coincide with an event you are directly associated with.

Urgent Bug Fix

If you've submitted an update to fix a critical bug in your app on the App Store and you are requesting an expedited review, be sure to include the steps to reproduce the bug on the current version of your app.

Time-Sensitive Event

For apps associated with an event, we recommend you plan and schedule the release of your app in iTunes Connect. However, if your app is stil in review and the launch of your event is quickly approaching, you can request to have your app review expedited. When submitting your request, it's important to include the event, date of the event, and your app's association with the event.

Please Note: Expedited reviews are granted on a limited basis and we cannot guarantee that every request will be expedited.

You can request for Expedited App Review from here