Restoring iOS without updating iOS in iTunes
You can here: https://ipsw.me/ find the old restore images (the image will be downloaded from Apple's server), which you from iTunes can use for the restore.

To use the downloaded .ipsw file, you hold down the "Option" key, then click "Restore iPhone" in the Summary page in iTunes. Here, you'll see a file browser menu pop up. Locate your downloaded .ipsw file, select it, then click "Open"

Just be aware that it is only signed iOS versions you will be able to install.

Here is a guide, that show the steps: https://ios.gadgethacks.com/how-to/downgrade-ios-13-back-ios-12-4-1-your-iphone-using-itunes-finder-0198642/