SFMC can export/import data two ways: files and/or API. SFMC has a connection with SFDC (MC Connector), so there is an OOTB API way of passing the data back and forward.

For real-time SFDC update: the form submission page that stores data in a DE can also write the data into SFDC (by creating or updating an existing object entry) via AMPScript functions. I'm assuming the form back-end page is hosted by SFMC.

For near-time SFDC update: if the for submission page is not hosted on SFMC, you can do the same as above in an hourly automation with a SSJS activity. Optimal for small amount of data submitted.

For once a day (or big amount of data): do a data extract of the DE and work with your SFDC people to export it via a file transfer to Sales Cloud and have the platform import it.