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Not certain what you are referring to here - Software Boutique or Synaptic or something else.
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I remove my old versions from the terminal with:
sudo apt-get remove --purge libreoffice7*
Since you did some manual install, it may not completely remove everything or possibly anything. You may need to manually remove what is left.
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Since you have a backup (Bravo - amazing how many do not), you at least have a safety net in case something goes astray.
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Edit:
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Just thought in case you did not know, LibreOffice is installed (Ubuntu 20) in the file system under opt