You would need to clean uninstall your old version before being able to run 2021. This would require you to uninstall both of them if you already installed the 2021.

You may Back up your Outlook by checking out this article.

Here is how you may Download files and folders from OneDrive or SharePoint to your PC

Help article here: Uninstall Office 2016, Office 2013, or Microsoft 365 from a PC

Please download this tool to remove old Microsoft Office license/s installed on your computer: https://aka.ms/SaRA-officeUninstallFromPC 

Uninstall Microsoft 365 guide here -  (Select Option 2)

Once you are ready to proceed, you may use the following steps as a guide.

Microsoft also recommends that you download this tool and should make it easier to remove leftover files. Download the Easy Fix tool.

Please go to you Local Drive: Then look for Program Files or  Program Files (x86) - the Microsoft office folder you would need to delete would be located on either of those 2 folders.

 You will be deleting Microsoft Office 16 or 15 if you have an older version like below.

After deleting the folder, you may restart your computer. Then reinstall the 2021 version. Click on INSTALL on the far right of the said version. 

Download 2021 version here: https://account.microsoft.com/services

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