Alt Text has a number of uses, included interaction with screen readers used by students with special needs.
Changing the Alt Text is a simple matter of enabling Alt text, and then selecting an item and clicking on the Alt text button.
Enable Alt Text:
Click on the settings icon located below to the title at the top.
Toggle the button to 'On' next to where it says 'Web Accessibility.
Add/Change Alt Text:
Select an item on your canvas.
On the action panel, locate the 'Alt Text' button.
Click it and note if there's text that's already there.
Type your new Alt Text, and press Enter on your keyboard to save.