H, The two URLs I include are examples of how one can approach fixing a problem or just learning more about how Ubuntu works. It's a stretch for any former Windows user to consider using a "Help" site to find the answer to a specific question. You know what happens if you try that in MSWindows, it's useless. I implore you to try using help in Ubuntu however, help.ubuntu.com is a wonderful URL IMHO. Rather than rattle on about how to change the server mirror your system goes to, to download software you request, I suggest you try searching it @ help.ubuntu.com. That said, I'll list three salient topics about installing new packages, etc.
Use this link to learn how to deal with all three topics
- Selecting the Best Ubuntu Mirror server for downloading software
- select the sources where your software center searches for packages you request
- learn how to manually add PPAs, which is a souce outside those offered by the standard ubuntu installation
Installing Grub, the first URL, is one I used an awful lot in the beginning of my Ubuntu education, and it "Sold" me on using this site to learn. The snapshot of selecting a different mirror to download from is made with the package/application/software (call it what you want) named "gnome-screenshot" it's a great tool, if you don't have it, but want it, install it, and learn how to use it.
The attached .png file shows how I typed the command incorrectly two times before finding the correct name of the program, 'gnome-screenshot'.
There is also a .jpg attachment showing the GUI of selecting a different Download-Server mirror. But if you go to help.ubuntu.com, you will see much better pictures of what the GUIs look like.