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(Just sharing some experience) Navigate to the directory where you downloaded the executable and run
sudo apt install ./displaycal_3.7.1.4-1_amd64.deb
It finished with the following warning:
N: Can't drop privileges for downloading as file '/home/<user>/Downloads/displaycal_3.7.1.4-1_amd64.deb' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
(not sure what this is trying to tell me) but launching displaycal
executed just fine. First I was prompted to update Agyll, which also worked.
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