1.) Log into your administrative account
2.) Go to Edit Website Content - UI Customization
3.) In the middle of the page are several links, click on Welcome Message.
4.) A popup should appear with a text editor.
5.) Make any changes to the welcome message, then click the Save button at the bottom of the popup.
6.) Your welcome message should instantly update on the home page. (If not, try Ctrl+F5 to refresh. Cache can sometimes interfere.)
Depending on your homepage design, you may or may not have an integrated welcome message area. If you have any questions regarding this, please log a support case.