All of the social icons that appear across your site are controlled by the "Social Links" variable.
One example of this would be your
social links that appear in the website
footer. These buttons require that the links
be stored in a variable file, just like the
one shown below:

To edit the data behind variables
such as this one, you will want to navigate to
Site Settings via the left-hand menu:

And then selecting the Site
Variables option from the sub menu

This area of the CMS will contain
all the variables being used for your
website.
Once you find the variable you are
wanting to change, in this instance the Social
Links variable, you can click into it to bring
up the file.
Once the file has been updated with
your new data, it can be saved. When saving a
variable please ensure the "Publish" checkbox
has been ticked

Now that the variable has been
saved, you will need to publish out both the
files and the whole site to ensure that your
new data is pushed out to all pages on your
website. Changes to variables may take up to
15 minutes to fully change following a site
publish.