How to update my version of MacOS 10.13.6 to 10.15 easily

It would depend upon your unmentioned Mac hardware age, and your present dependency upon 32-bit applications that are not supported in Catalina (10.15.7).

Visit the Apple () menu > About this Mac panel and review what is in the parenthesis on the product description line. It will be a {early, mid, or late} modifier, the year, and the Mac model). Compare that to the hardware requirements at How to get old versions of macOS using strictly the Safari browser. If your Mac meets the hardware requirements for Catalina, then there is a link to obtain the full installer from the Mac App Store, which will download to your /Applications folder.

I recommend that before you pursue the second paragraph that you download and run the free Go64 application that will show you a list of 32-bit applications that are not supported on macOS Catalina due to its strictly 64-bit application support. Some 32-bit applications may no longer be supported by their developer, may now be subscription-ware, incur a vendor 64-bit tax, or may just be a free 64-bit update of the application. You will need to confirm with each vendor for those applications you will need on Catalina. Similarly, any printer/scanner that you are currently using with High Sierra will need you to check on the vendor site to see if there are drivers available for macOS 10.15.*. You don't want to upgrade and then find that you have paperweights with no driver support.

Finally, the last thing you should do before any upgrade is to remove any anti-virus or so-called cleaning applications, and then perform a last High Sierra Time Machine backup. Do not reuse this drive for Catalina Time Machine backups.

If you are currently using Office 2011 for Mac, whether you purchased it or not, it is not supported on Catalina, and you will need to obtain a new Microsoft 365 or Office 2021 for Mac license and the relevant software. You document files are unaffected by the 64-bit issue.

Doing an upGrade to a newer version of macOS without Exit / Recovery Plan is IMHO - a possible invitation for a long and time consuming recovery process.

Food for thought

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site