In a serverless way, i.e. from a static html page with no server-side backend, you could create a lambda function in the supported language of your choice and with the proper IAM role, to move the data from one table to the other - your html page could call it via an API call, and I would suggest you use AWS API Gateway to expose an api endpoint that then calls the lambda function.
If on (one of the other many) other-hands, say you were using ASP.net or PHP on the server side, you could use the AWSSDK to talk to the dynamodb directly and accomplish the same thing.
Besides these two options there are many, many alternatives and variations - and with all of the options you are also going to need to deal with authentication/security to make sure no one can make calls to your database/service that they aren't permitted to - perhaps not important for your demo application, but will definitely be an issue if/when you go live.