In Windows, for permanent installations.
The logic is.
If Gimp 2.10 installer finds a Gimp 2.8 installation it considers it an update and will install to the same location as Gimp 2.8 Usually C:\Program Files\GIMP 2\ and you have no choice as to location.
No good just deleting the old Gimp 2.8 installation, the details are still in the Windows registry and Gimp 2.10 will not install.
If you just have a Gimp 2.8 installation and want to keep it.
Edit the Windows Registry to remove the Gimp 2.8 uninstall information. Use the Gimp 2.10 installer, use the customise option, install to a location such as C:\Program Files\GIMP 210\ using the customise option with the installer
example https://youtu.be/dRsk29KPez4 about 5 minutes.
There are variations on this. Backup Gimp 2.8 folder, Uninstall Gimp 2.8, Install Gimp 2.10 to C:\Program Files\GIMP 210\ Restore Gimp 2.8 backup.
or
You can re-install Gimp 2.8 following a Gimp 2.10 installation by renaming folders before and after installations - very messy but works.
example https://youtu.be/L9oP_MjGYVA about 5 minutes.
The alternative, similar to Zorba, but use a portable Gimp 2.10 with your existing Gimp 2.8 installation.
The logic is.
If Gimp 2.10 installer finds a Gimp 2.8 installation it considers it an update and will install to the same location as Gimp 2.8 Usually C:\Program Files\GIMP 2\ and you have no choice as to location.
No good just deleting the old Gimp 2.8 installation, the details are still in the Windows registry and Gimp 2.10 will not install.
If you just have a Gimp 2.8 installation and want to keep it.
Edit the Windows Registry to remove the Gimp 2.8 uninstall information. Use the Gimp 2.10 installer, use the customise option, install to a location such as C:\Program Files\GIMP 210\ using the customise option with the installer
example https://youtu.be/dRsk29KPez4 about 5 minutes.
There are variations on this. Backup Gimp 2.8 folder, Uninstall Gimp 2.8, Install Gimp 2.10 to C:\Program Files\GIMP 210\ Restore Gimp 2.8 backup.
or
You can re-install Gimp 2.8 following a Gimp 2.10 installation by renaming folders before and after installations - very messy but works.
example https://youtu.be/L9oP_MjGYVA about 5 minutes.
The alternative, similar to Zorba, but use a portable Gimp 2.10 with your existing Gimp 2.8 installation.