Method 1 - Control Center:
1. iOS 11
Step 1: Swipe up the Control Center and tap the Bluetooth icon.
Step 2: The Bluetooth is turned on when the Bluetooth icon is in blue color.
2. iOS 10 and older
Step 1: Swipe up the Control Center and tap the Bluetooth icon.
Step 2:The Bluetooth is turned on when the Bluetooth icon is in blue color.
Method 2:
Step 1: Go to Settings on your Home screen.
Step 2: Go to Bluetooth
Step 3: Toggle on Bluetooth