Modifications done in a module, in the theme, in CSS or in Javascript cannot be automatically detected so you need to clear the cache to see the changes.
There are 3 ways to clear the cache
In the configuration of the module "Page Cache Ultimate", go in "Statistics" menu. You can filter pages that are concerned by your modifications. Then click on the button "Clear cache" at the bottom of the table.
Click on the button "Clear cache" in the configuration of the module "Page Cache Ultimate" on top and right of the screen, this will clear the cache all pages.
Use the URL that you will find in the "API" menu in the configuration of the module "Page Cache Ultimate"