how to install older versions of Adobe Robohelp?

You are saying you have downloaded and the installed the trial version of RoboHelp 15 and then asking how to download the older version. 

Are you meaning you have downloaded RoboHelp 2020 as that would be the logical number but it is not known by that number?

The older versions are not available from Adobe. You might find them on dodgysupplier.com but I wouldn't recommend it.

Getting used to the new version does take a little while but once you have done so, you will find it is much easier. I now dislike having to go to the old versions. There's a lot of explanations about using the new version on my site. RoboHelp Tour Introduction (grainge.org)

It occurs to me that if you are more comfortable with the older versions, you already have a licence, no? If that is registered you will be able to download it again. What you can't do is buy it.

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See www.grainge.org for free Authoring and RoboHelp Information