You can now change the current avatar on your text testimonial
Once you are in your space, click the text content or the ⌄ icon at the bottom-left to expand the testimonial card.
Once the card expands, you will see a list of icons at the bottom, and you will find the Edit icon. Just click it.
You will see the following popup modal. Scroll to 'Update avatar', upload the avatar you want, and then click the 'Update the testimonial' button to save the changes.
Sample output once it changes.