Q: Iam using iPhone 4s after updating my iphone to iOS 8 I can't find personal hotspot option.where is hotspot option?

Did you already try the iOS: Troubleshooting Personal Hotspot ?

Did you verify that your carrier supports Personal Hotspot for your device and that you have Personal Hotspot enabled on your cellular plan?

• iPhone: View a list of carriers that support Personal Hotspot on the iPhone.

Did you reset the network settings in Settings/General/Reset/Network settings?

If the carrier does not support the Hotspot feature, it will not show up on your phone, this is carrier related and no software problem.

Did you already try this?

iOS: Updating your carrier settings

You're welcome, enjoy your phone