Your system admin can decide to turn off the ability to switch between Lightning and Classic. If you are not seeing these options, contact your system administrator.

If you are in Classic, you can click "Switch to Lightning Experience"

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This guide provides a quick and easy way to switch between Salesforce Classic and Lightning Experience. By following these steps, users can seamlessly transition between the two interfaces, allowing them to leverage the benefits of each and enhance their overall Salesforce experience.