If you are determined to try and believe you have the necessary "blobs" or a compatible jailbroken state, the general (highly technical) outline is below.
Get the iOS 7.1.2 IPSW file for iPhone 5s (model A1453, A1457, A1518, A1528, A1530) from a trusted IPSW archive. how to downgrade iphone 5s to ios 7
You must have a .shsh or .shsh2 file corresponding to your ECID and the target iOS version (e.g., iOS 7.1.2). Without this "ticket," Apple's server (or the local simulated server) will reject the install. If you are determined to try and believe
You'll need to have SHSH blobs saved for iOS 7 for your device. These blobs are crucial for the downgrade process. Tools like TSSSaver (by tsschecker) can help you save or fetch these. Without this "ticket," Apple's server (or the local
Apple has stopped signing iOS 7 almost a decade ago. This means a "standard" restore via iTunes or Finder is impossible. You generally have two scenarios: