When you're in the Measure Tool, select the User Tools Library icon on the left hand side and select the VIEW USER TOOLS button. This will take you to your User Tools Library where you can view, edit, or deactivate your custom made Tools.
Firstly, find the Tool you wish to edit and click on it, this will open up a popup with all the details relating to this Tool. Here you can view, edit or remove certain details relating to the specific User Tool you've selected.
Once you've reviewed all the details and are happy with any changes you've made, simply click on the UPDATE button.