I want to upgrade my Peazip. Do I need to uninstall the old version and then run the deb file or can I just install it over the old version?
As a general rule of thumb, I always remove and then purge an old copy of an app before installing again or installing a new version
sudo apt-get remove ***
sudo apt-get purge ***
sudo apt-get autoremove
sudo apt-get update
Then install the deb with gedebi
Thanks. Is your method more thorough than total removal using package mgr?
To be honest, I am not sure Fixit7. But, it’s what I have always done.
Hi @fixit7,
if you don’t remove one package before installing a newer one (talking about 3rd party .deb packages here not stuff from the Ubuntu repo!), you will end up with 2 versions of the same app with different numbers, as @stevecook172001 said, remove all traces of the other one or you might end up confusing yourself!. 
I used Package Manager and did a complete removal followed by the new installation.
I wonder how you can have two versions since they would have the same executable?
Unless they are put in separate directories.
If the package has the same name, you can’t. it will replace the older version, not install itself alongside (even if it changes directories between two versions for some reason, it will remove the old ones when updating).
BTW, you really don’t need to uninstall a package if you want to install a new version manually unless both versions are packaged differently (e.g. one version having two or more packages because it splits the binary and some data or plugins, and the other version having a different number because e.g. it puts everything in one).
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In my experience, that is not always true. When a new package is installed, sometimes it will not overwrite and, instead, rely on the existing config files of the old installation if they already exist. Existing config files that may not work properly with the new installation for a variety of (often obscure, in practical terms) reasons. That is precisely why I not only remove the old application, but also purge it in order to remove any remaining config files. That way, the new installation will also be guaranteed to install new config files.
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I have a doubt: isn’t APT supposed to replace the config files, unless they’ve been modified, in which case it asks the user?
Not always in my experience. I can’t claim to understand the underlying process involved. But, I can state I have experience of the above outcome. Hence my rule of thumb approach
My take away is to always uninstall a package first.
I have noticed that when doing a total uninstall, there a still residual files left over.
Like manuals. Looking at file dates helps in seeing what is still being used.
They are usually small, but they do add up over time.
“If it isn’t necessary, I want it gone.” 
I just manually delete them.
If you purge a package (not just uninstall it), there shouldn’t be residual files at system level. Or are you talking about settings in your home dir? If that’s the case, it’s normal, APT will never remove anything in the user’s personal files.
Yes in my home directory. For ex. I just found a frozen bubble dir there.
Everything that is in your home dir is yours to manage, as the files there are your personal data, the system (apps, root user…) has nothing do do with them. It’s by design. Can you imagine APT deleting e.g. all your .odt documents if you uninstall LibreOffice?
I don’t think Fixit7 is necessarily complaining that removing an app does not remove stuff from one’s home folder. But, the point is, some of that stuff is not user generated files. Some of it is application config system files that may still have an effect on a new installation of an app.
So for example, I recall that when I have re-installed Wine, if I do not remove any and all Wine stuff from my home folder, the new installation does not always work as expected.
Steve,
You are exactly right. 
Andy
For me, it would not be a problem for at least 3 reasons.
I keep all odt files in their own separate directory.
I keep daily backups of them as well in a separate drive.
I make frequent image backups in a separate drive.