I actually want to use my mobile to download updates as my internet is not so fast and I have to keep my computer running for a long time to install them. I want to download them in my mobile and then install later by transferring them to computer.
2 Answers
For a situation like the one you described, I think that synaptic will be appropriate for your purpose.
(I had the same scenario you described, dial up connection at home, and high speed connection at my office.)
Short Version
Use synaptic to create a shell script in the slow machine. Run the script in the fast machine to download packages. Copy back the downloaded packages to the slow machine. Use synaptic to install the packages.
Long Version
If synaptic is not installed in your machine, you will have to install it.
sudo apt-get update
sudo apt-get install synaptic
I understand that your computer has very slow speed. But I hope that it can cope with the update and around 2500 K download for the above installation.
Start synaptic,
sudo synaptic
You get the opening window of synaptic. Select the files you want to install using the Search function.

Marks the packages you want to select, either left mouse click or right mouse menu-select. Finish marking all the packages you want.

From the File menu generate the download script.

Save the script at some convenient location. Use any file name you like. Let us say your
name is mydownloads.sh

If you close synaptic, you can ignore the warning about not installed packages.
The script is a simple shell script, containing a series of wget commands, like the following,
#!/bin/sh
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/c/cadubi/cadubi_1.3-2_all.deb
Copy the script to your mobile, start a terminal, move to the directory where you copied the script and run it there.
sudo ./mydownloads.sh
Please remember that wget needs to be installed in that device. If it is not installed, install
sudo apt-get install wget
Once finished running the script (it may take some time depending on the Internet speed and volume of download), you will get a number of .deb files in the directory from where you run the script. Copy those files back to your computer and run synaptic again.
sudo synaptic
From the file menu, use the Add downloaded package function to select the directory in your computer where you copied the downloaded files.

Click on Apply. Installation of your downloaded packages should go without any problem.

From time to time, you will need to run
sudo apt-get update
at your slow speed machine, select Mark All Upgrades in synaptic and then go through the process of generating the script, download the files at your high speed machine, and
then install the downloaded files to keep the slow machine upgraded.

- 3,203
Keryx is one possible solution to your problem.
With Keryx you can decide what to download (updates included) and then download them on another computer.
If Keryx doesn't work as advertised, there is a step by step procedure for using Keryx v0.92.4.1