How do I erase my laptop air if someone else has it

Read this support article about what to do if your Mac is lost or stolen. Click here --> If your Mac is lost or stolen - Apple Support

Monicamarieeming wrote:

Please help, my laptop was either stolen or misplaced. How do I erase my personal content for security and reset to factory settings

Follow these instructions: If your Mac is lost or stolen - Apple Support

You can erase the Mac with Find My if it was active, but it'd be a bit counterproductive if FM reinstalled the OS too. Then the thief would have a Mac ready to setup and claim as his/her own.