Hello, and thanks for the question. You'll need to download the file and use a 3rd party site such as https://gotoes.org/strava/index.php or https://www.fitfiletools.com/ to alter the date/time and then upload the file manually. You will also have to delete the original activity so it's not duplicated. Let me know if you have any other questions about this process

Hi, how do I download the file? I cannot see this option next to my activity in the app? 

You can only download from the Desktop (web) version of Strava. It's in the "..." menu on the left side, then "Export Original."