We advise against changing your username. Doing so will change the address of your public profile, break existing web links to your profile, and impact search engine results for your uploaded photos and ideabooks.
If you still want to change your username, first sign in to your account. Go to Your Houzz, then click on Edit Profile. In the Account Details section of this page, click the Change Username button. From there, you will be able to enter a new username.