Actually, the instructions are pretty straight forward.
before upgrading/uploading, backup your database and files.
Yes, the "existing installation folder" is where it was originally installed. Upgrading is merely overwriting the original files from a previous install. The folder is not named in the instructions as the folder name can be changed from the default directory name.
You don't upload all the files to the "install" directory. The install directory is where the install/upgrade scripts and sql files are placed, to perform an install or upgrade. Once you upload 4.5.1, simply login to your WHMCS, and it should automatically invoke the upgrade scripts. Once the upgrade is done, simply delete the install directory (for security reasons)
If you can't figure it out, WHMCS does provide an installation/upgrade service at a reasonable price.