But am I gonna loose all my files and clients?

No, you will not lose all your files or clients.  The common folder which carries over from 2017 must have become damaged on initial install.  But you shall have no client loss.

How can I re-download the 2018 version?

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@MT wrote:

How can I re-download the 2018 version?

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This article should help you out! 

https://proconnect.intuit.com/community/help-articles/help/proseries-download-and-installation-faqs/...