Please help me with this. I need my phone battery working at least eight hours.

You can buy yourself a small power bank so you can extend the battery life even if you're on the move and double or triple it, some are very small or a magsafe power bank can stick to the back of your iPhone

you can turn off some features that can drain the battery like in Settings > General > turn off Background App Refresh, you can turn off in Display&Brightness Always on Display and Raise to Wake.

Go to Settings>Battery>Battery Health. What does it say next to "Maximum Capacity"? If it is 80% or less, you need a new battery.

iPhone Battery Repair & Replacement - Apple Support

Yes you can actually change the battery, Apple suggests to replace it when it's dropping below 80%

The magic number is 80% for a battery replacement.

The reading shows 86% . Should I change my phone battery?

Replace the battery at an Apple Store or Apple authorized service shop when it is down to 80%. You probably have another 5-6 months on the current battery.