How to sort out the issue occuring while downloading itunes?

Prior to installation, make sure you have at hand the name and administrator password. Data need to install both iTunes. Make sure you have enough disk space. Even the smallest applications may require hundreds of megabytes to complete the optimization phase of the installation.

Check your hard drive before installation. You may want to use Disk Utility to confirm that the disc is in good condition and that the permissions are correct. Do not use the computer while installing an update. Although this is not mandatory, you may wish to avoid using the computer during the upgrade process. It is also recommended that you restart the computer just before installing an update. If other applications are running, turn it off to have as much memory as possible during installation.

Find out what version of iTunes you have installed.
Open iTunes.
From the iTunes menu, choose About iTunes.
A window with scrolling text. The first line shown indicates the version of iTunes (press the spacebar to stop the text if it moves too fast and do not have time to read it).
Download the latest installer
Open Safari or another browser.
Go to the iTunes download page and click the Download Now button.
Make sure you selected the Mac OS X.
Click the Download button to start downloading.
Once downloaded the file, a new Finder window containing two elements (see below). If the window does not appear in the Finder, locate the file iTunesX.dmg (where X is the iTunes version number) on the desktop (or wherever your downloads go) and double click it to display these items:

A "Readme before installing iTunes." This document contains system requirements for the version of iTunes and possibly also other useful information about the installation.