How to switch to lightning?

1. Enable My Domain

Enabling “My Domain” is a prerequisite to switching to lightning. In order to enable your domain, go to Setup->My Domain.

Enter your custom domain name, click on register. Once you register, it will look like this :

2. Check if Lightning is Enabled for your Profile

In order to switch to lightning, the “Lightning Experience User” checkbox must be checked on the profile of the user.
To enable this permission, go to Setup->Profiles and under your profile check the “Lightning Experience User” checkbox.

3. Click on the “Switch to Lightning Experience” button