Can we change the sequence of tabs in lightning. If yes how can we change the order ? It was not possible till winter 15.
Yes you can do that, see the picture below. To do this, you need to upgrade the app to a Lightning App. As per Salesforce "Upgrading a Classic app to a Lightning app lets you and your users take advantage of custom branding and the enhanced navigation features available in Lightning Experience."
Refere to this trailhead for details. https://trailhead.salesforce.com/en/lightning_apps/lightning_apps_upgrade
Image Source:- Salesforce Trailhead