After changing the code in the file, how to force the user's browser to update the cache itself?

Question

PHP

Hello!
After the server was launched, it became necessary to change some styles and pictures, but the clients who had already accessed the site before the changes see it in the old style.
How to force the browser to update the cache for a given site without manual intervention on the client side?

Answer 1

href="style.css?v=<?= time()?>"

Answer 2

Use etag