My available space in my iPhone storage just keeps on reducing

Your tag line indicates that you have an iPhone 6s running iOS 15. You can only update an iPhone 6s Plus as far as the latest release of iOS 15. You cannot update it to iOS 16 or higher, no matter how much free storage you have.

If you are trying to update to the latest release of iOS 15, and your phone has only has 400 – 500 MB (0.4 – 0.5 GB) of free space, that isn't likely to be enough. You will likely need to offload some data to a computer, or just delete it outright, to make enough space to install the iOS 15 update.

Note:

• When you delete photos and videos, you might not get space back right away. That's because the phone will put them into a Recently Deleted album so you can change your mind. You can free space by deleting them from the Recently Deleted album (before the system empties the trash in 30 days) - but take note: any deletions from this album are permanent deletions. Afterwards, you will not be able to get the photos and videos back.
• If you are using iCloud Photos, and have sufficient iCloud space, you may want to use "Optimize iPhone Storage"

See also:

Updating iOS: common issues and solutions - Apple Community