, if at all possible or advisable.
Yes it's possible, but if I recommend installing High Sierra on a completely separate disk, to avoid altering your existing Mac in any way. To learn how to do that please read Upgrading macOS without fear.
If you want to install HS on a separate partition of the Mac's built-in storage then you need to create that partition first by booting macOS Recovery, followed by partitioning the disk, followed by installing HS on it. Read About macOS Recovery - Apple Support and Partition a physical disk using Disk Utility - Apple Support.
In either case, the salient point is to select the proper installation volume so as not to inadvertently overwrite your existing installation:
Click that and be sure it's installing where you want it to install.
Don't do anything without a backup. To learn how to use Time Machine please read How to use Time Machine to back up or restore your Mac - Apple Support.
Please note that whatever you decide to do, you will need to obtain an external hard disk drive. They're cheap enough.
