F5 typically refers to the key on a keyboard used to 'Refresh' a computer screen. For example, in the case of a web browser.
There are no categorical antonyms for this word, although one could loosely refer to the acronym AJAX as an antonym for F5.
AJAX refers to a set of Web technologies on the client side to create asynchronous Web applications.
Here's a list of antonyms for AJAX.
Opposite of web technology