The Welcome text is a little buried, but fortunately you only need to change it once. To edit the welcome page, you need to navigate to Structure > Blocks > Seven
You can also replace the XXXX in this URL with the number of your site (note: you will need to be logged in as an admin for this to work).
From there, scroll to the botom of the page and you'll see home page welcome block for visitors.
Click the configure link to the right and you will get an editable text block.
Change the text and click the save block button at the very bottom of the page and your text will be saved. Note: you need to be logged out to be able to see this page.