What they meant is turn off FMIP (Find My iPhone). They can't fix or repair your phone if it's still signed in or connected with your Apple ID.
Go to iCloud.com on a browser, sign-in with your Apple ID & password. Click on "Find iPhone" then click on "All Devices" at the top part of the screen. Select your phone on the list of devices & select "Remove from account" and you're done.
You can bring your phone to the nearest Apple Store or Apple Authorized Service Provider to get it repaired. Book an appointment on getsupport.apple.com or use the Apple Support app. If there are no available appointments, it would be best to call the store directly to book an appointment. Or you can call Apple support - Contact Apple for support and service