I guess you have to contact the product team as written on the download page:
How do I get the Excel Add-in?
The Excel Add-in version 2.6 is no longer available to download. You can benefit from improved speed and new features if you download and install the latest version of the Excel Add-in. The write-back function offered by version 2.6 will be included in future versions. If you need this feature in the interim, contact our Product team to discuss options.
If needed I still have the 2.6.208 installer file if you need it....
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