Line 181 of your pastebin:
<div style="float:right; width:227px; height:31px; margin-top:30px;"><img src="https://www.sgestates.co.uk/wp-content/uploads/2012/05/tel.png" alt="*" /></div>
It is up there as an image, rather than text. You would need to either create an image with the new phone number, upload the image, then change the path accordingly
-OR- (and I would suggest doing this rather than the above)
Create a new container and delete the above from the header.php
<span id="header-tele">
Insert new phone number here.
</span>
Then add this to the bottom of your CSS file
#header-tele
{
font-size: 36px;
position: absolute;
color: #2c2f91;
right: 10px;
top: 36px;
font-family: Times;
}
But I agree with Esmi, this probably is not the best place to ask this question, and although I suggested a fix, it would be better to check with whoever built the theme.