I currently have MAC OS High Sierra Version 10.13.6. How to can I upgrade to 10.14.

Without knowing the model year of the iMac we don't even know if that is possible. In order to run 10.14 your iMac must be a 2012 or later iMac. If you own a 2011 or earlier iMac then 101.3.6 is the end of the road.

If you own a 2012 or later iMac then use How to get old versions of macOS to download 10.14.

Finally, if you don't know the model year of the iMac, please click the Apple symbol in the upper left of the display and then click About this Mac and you will find the model year.

Your welcome, considering a 2010 is now 12 years old, have you considered replacing it with a more up-to-date Mac? You are missing out on a lot of new functionality and speed with a newer Mac. In addition, the longer you keep old technology the more difficult it becomes when it is time to upgrade as older legacy software loses support, getting parts when the computer needs them (the 2010's were classed as obsolete many years ago now) and there are many other factors to consider.