Hi,
First of all I assume you do have a complete backup. Remember to
include the users and access files from the admin section (C:\Program
Files\ReposServer\admin). Upgrades do not rely on the backup, but it
should always be there.
Basically you just uninstall the old version and install the new one.
The uninstaller keeps files that might contain custimizations. I don't
remember exactly how it worked in 1.2.4 though.
To be sure that old config files are replaced, after uninstall:
manually delete from the program folder all files you have not edited
yourself.
If you don't have any changes in apache configuration or html
contents, you may remove everything but admin\repos-users and
admin\repos-access.
Alternatively backup/rename C:\Program Files\ReposServer and do a
fresh install, after which you can copy the user files to the new
program folder.
The Repositories folder might need some special treatment for a major
upgrade (maybe that's why your earlier upgrade attempt failed). If
inside it you have "dev-repository", rename it to "dev". This naming
was introduced quite late to support the SvnParentPath multi-repo
config.
After that install with the same parameters.
/Staffan