What are the change made from 39 version to 40 version in Salesforce?

Hi Raghav

Here are the few highlights of salesforce API version 40

1-Summer ’17 Salesforce Classic Only Highlights

• Lightning Experience Migration Assistant
• Update existing Letterhead email templates by changing which letterhead is associated with the template.
• Stay-in-Touch feature officially retires. The Request Update button will be removed in Summer 17.
• Cloud Scheduler retirement: Unavailable in Salesforce orgs created in Winter ‘17 or later. If you enabled Cloud Scheduler before Winter ‘17, it remains available until Winter ’18.

2-Summer ’17 Salesforce Lightning Experience Only Highlights

• Search Enhancement: Find the right opportunity by searching for opportunity name plus the account name.
• Manage Records Using Split View for Console Apps. When you have a console app, rather than show a list view as a table, you can select to show the list view as a split view. This allows your reps to work with multiple records at a time.
• Rich Text Editors Reenabled for LockerService. Previously, when LockerService was activated in a sandbox or personal DE, the rich text editor became disabled.
• “Disable Access to Lightning Experience and the Salesforce1 Mobile Browser App from IE11” Critical Update Canceled

3-Summer ’17 Salesforce Classic & Lightning Experience Highlights

• Remove the ability for users to switch back to Classic. By enabling the “Hide Option to Switch to Salesforce Classic” on the profile or permission set, this disallows the user from switching between Classic and Lightning.
• Orders are now available via global actions
• Person Accounts now shows all notes
• Find duplicates between contacts and leads

More new changes you can find by going through Salesforce Summer '17 - Salesforce Release Notes

Hope this may help you

Thanks