Ubuntu – Download deb packages with download manager instead of apt

I think it's safe to remove the archives files. So if you want to free up disk space this are my recommendations:

To delete downloaded packages (.deb) already installed (and no longer needed)

sudo apt-get clean

To remove all stored archives in your cache for packages that can not be downloaded anymore (thus packages that are no longer in the repository or that have a newer version in the repository).

sudo apt-get autoclean

To remove unnecessary packages (After uninstalling an app there could be packages you don't need anymore).

sudo apt-get autoremove

To delete old kernel versions

sudo apt-get remove --purge linux-image-X.X.XX-XX-generic

If you don't know which kernel version to remove

dpkg --get-selections | grep linux-image

Source: Limpiando Ubuntu: comandos y programas (actualización) & How do I free up disk space?

You have two options here.

The Queue-Mode configuration option

The default, "host", creates one outgoing connection for each host. If you set it to "access" it will create one connection per URI type (which I understand to mean, one for http, one for ftp, and so on - since most connections are http, this means it'll access each server sequentially).

One way to do this that will also apply to update-manager is to:

echo 'Acquire::Queue-Mode "access";' >/etc/apt/apt.conf.d/75download

if you want to try it with apt-get once to see if it helps:

apt-get -o Acquire::Queue-mode=access update

The Acquire::http::Dl-Limit option

This is similarly to reduce the bandwidth used to something your connection can handle (and as per the manpage, "this option implicit deactivates the download from multiple servers at the same time").

echo 'Acquire::http::Dl-Limit "70";' >/etc/apt/apt.conf.d/75download

or

apt-get -o Acquire::http::Dl-Limit=70 update