How to revert to High Sierra iOS?

High Sierra is a Mac OS, not iOS. iOS is for phones and tablets. See this tip on downgrading Mac OS:

How do I downgrade my Mac OS X operating … - Apple Community

Keep in mind, any converted libraries under a new OS can not be read. The data files still can be. But album and playlist information is lost once converted to the new OS. If you have not modified your Time Machine or clone backup you made prior to upgrading to Monterey you are better off restoring the OS and the backup than trying to access files already upgraded.

You cannot install any operating system older than what Apple originally shipped on the Mac. If you have sufficient hardware resources, you may be able to install High Sierra into a Virtual Machine guest and run it that way.

Build a bootable installer:

Create a bootable installer for macOS - Apple Support

Then boot it, and use it to erase and install:

Erase and reinstall macOS - Apple Support

If this is an Intel Mac, acquire and install a hypervisor (“virtual machine”) and install macOS 10.13 High Sierra as a guest. Most hypervisors will support virtualizing macOS on macOS on a Mac, but check that with the particular hypervisor vendor to confirm.

atul_aggarwal wrote:

Thanks.

The computer is older than Sierra.

It used to have Sierra, then over time it got upgraded.

I don't have any backup, TimeMachine etc.

No backups? That means the data is considered of little or no value, or easily regenerated, which makes this easier.

But again, you cannot install a version of macOS older than whatever the target Mac was first shipped with.

You must either update the apps and contents to more recent, or can potentially install and use a hypervisor (or maybe both a hypervisor and emulator) as a delaying tactic.