Why my softwhere update stuck on Estimating time remaining?

Because it’s still estimating the time remaining to download the update. The time it takes is dependent on your Internet connection speed. If you want to see if it’s working to got Settings/General/iPhone Storage and find the update download. Write down it’s size. Then wait 10 minutes and do it again. If the size hasn’t changed perhaps it is stuck; delete the download and start over at Settings/General/Software Update. Also note the following:

Here are the conditions that must be met to update iOS:

• You must have a fast Wi-Fi connection; you cannot update over cellular data
• The phone can not be in Low Power mode
• The battery state of charge must be above 50%
• You must have at least 3 GB of free storage
• You cannot be connected through VPN
• If you are not planning for it to update automatically you should turn this option off, as it will only update when the phone is plugged in, connected to Wi-Fi and locked
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).