1.
Make sure you do not have ArcMap or ArcCatalog open.
2.
As a precaution make a backup copy of your existing C:\RivEX folder. This can be done simply by right clicking on the folder in File Explorer, select copy and then paste this folder back into C:\ with whatever name you choose.
3.
Download the zip file from the link below to a location you can find, I suggest C:\
4.
In File Explorer double click on the downloaded zip file and extract everything to C:\
Do not extract into the existing C:\RivEX folder.
5.
You will be asked if you want to overwrite the existing folders and files, say YES to this. If you are not asked to overwrite existing files then you are extracting the files to the wrong location, they must be extracted to C:\ so they are updating into the correct sub folders.
6.
After upgrading if you find that the Help file is blank then you need to unblock the CHM file, this may or may not occur depending upon your system setup. Advice can be found here.