I am currently running macOS Sierra 10.12.6 on my Mac. I am in the process of upgrading the OS to the current public release macOS High Sierra 10.13.6. Will I lose my data during the upgrade process?
Generally speaking, upgrading to a subsequent major release of macOS doesn't erase/touch user data. Pre-installed apps and configurations too survive the upgrade. Upgrading macOS is a common practise and carried out by lot of users every year when a new major version is released.
However, you should be cautious and be responsible for the sanity of your data. Upgrades doesn't negate the possibility of going wrong, and in the unlikely case, may cause damage to the hard drive, thereby causing you to lose access to data.
It is highly recommended to keep properly working data backup when doing a sensitive activity like OS upgrade.
I'll put extra emphasis on having a backup that works! A backup that doesn't work is as good as no backup.
Ensuring you have proper power backup and uninterrupted Internet connectivity, you should be fine with a high degree of probability. Having a backup ensures complete peace of mind :)
I have upgraded a lot of macOS devices (many from back when it was still OS X) and through both minor and major updates I have never lost any data.
What you can lose are certain applications that were not updated and are no longer compatible with the new version. If you have data that can only be read through these applications, your data can be effectively "lost", even though it is still there. This is the one thing you should check carefully.
No, as @nimesh-neema said, you will not lose your data but it is both a sensible precaution and good practice to back up your data.
Occasionally an application may be replaced (or updated) with the upgrade and then you may lose settings associated with the application, or the new application will have only default settings, which is small consolation as it can still feel like you've lost data.
But there is always the risk of a glitch or corruption in the upgrade process and, after the upgrade, you are presented with what looks like a "clean" installation - without any of your files or folders. In most cases these are still on the disk (somewhere ;-) ) and can be recovered, but it is usually easier to reload from a backup (because you know where that is and don't have to hunt for anything).