It requires 8.18GB available space I have 8.33GB still unable to update

It will require a bit more space than the actual capacity, to ensure that the update runs flawlessly.

If you can not free up more space, then I would recommend connecting your iPhone to a PC or Mac and updating it through. It does not require space on the iPhone as the update is downloaded and installed from the computer.

You can read here how to update from PC / Mac:

Update your iPhone, iPad, or iPod touch - Apple Support

It actually requires more space than what is stated for "overhead".........about 15-20% more on average