Download: https://www.fopnu.com/download/linux.html

.DEB

I read that the commands should be:
wget https://download2.fopnu.com/download/fopnu_1.57-1_amd64.deb;sudo dpkg -i fopnu_1.57-1_amd64.deb
sudo dpkg -i fopnu_1.57-1_amd64.deb
sudo apt install -f

But that not worked:

wget https://download2.fopnu.com/download/fopnu_1.57-1_amd64.deb;
2022-01-26 22:00:10 (162 KB/s) - ‘fopnu_1.57-1_amd64.deb’ saved [9085328/9085328]

sudo dpkg -i fopnu_1.57-1_amd64.deb
Selecting previously unselected package fopnu.
(Reading database ... 212217 files and directories currently installed.)
Preparing to unpack fopnu_1.57-1_amd64.deb ...
Unpacking fopnu (1.57-1) ...
Setting up fopnu (1.57-1) ...
/var/lib/dpkg/info/fopnu.postinst: line 4: gconftool-2: command not found
/var/lib/dpkg/info/fopnu.postinst: line 5: gconftool-2: command not found
/var/lib/dpkg/info/fopnu.postinst: line 6: gconftool-2: command not found
Processing triggers for desktop-file-utils (0.26-1) ...
Processing triggers for mailcap (3.69) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...

$ sudo apt install -f
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.

After "apt search gconftool" i have "apt install gconf2". Running that dpkg and apt install -f again did not caused "command not found" and mentioned apt returned "0 newly installed", yet "whereis fopnu" shows /usr/bin/fopnu and it was added to the programs menu and launched OK apparently.

To upgrade, i need to run same steps again and old files will be overwritten?

Is there better way to run Fopnu on Debian? (not found in official repository)

I'm on Debian too. Yes. To upgrade, you basically do the same thing again. Your fopnu config folder is(~/.fopnu/) and it won't be overwritten when you upgrade.

I've never used the deb. Here's what I did.

1) I downloaded the .tar.gz, unpacked it and moved the fopnu binary to ~/bin/

2) For the fopnu.desktop file, I moved it to: ~/.local/share/applications/

3) In the fopnu.desktop file, I changed the exec line in the fopnu.desktop to "/home/username/bin/fopnu %U"

4) I moved the fopnu.png file to ~/.icons/  if you don't have that folder, make it.

5) run gtk-update-icon-cache