Currently, undo tool is available only on mobile apps.
There is no undo tool on Web Browser.
However, anything you add to an image or video can be removed in any app.
*Note that mosaic can't be removed once you save it.
Please refer to the following manuals about using the marking function:
▼Add markings and edits to images, such as arrows
▼Mark the images with arrows, etc. (iOS version)
▼Marking pictures with arrows, etc. (Android version)
▼About the image edit function (Windows Desktop App)