"Flex fare" options for flight needs that may change last minute?

In June I will be heading to Cape Canaveral to attend a rocket launch. As you may know, rocket launches are given a "Launch window" and can be pushed back a day or two or three or... you get the idea.

I just found out the launch date for this mission I want to attend, and plan to book a flight! If all goes as planned, I fly in the day before launch, see it launch on a Saturday, and leave on Monday.

However I want to be sure that I am covered in two rather likely scenarios:

1. Launch is delayed by a few weeks/months before I leave.
2. Launch is delayed by a few days after I arrive in Florida.

I know flexible fares are a thing, but I don't see any clear options on how to book them. Doing a Google search for "Book flexible fare" seems to mostly lead to travel aggregate websites that advertise "Flex" in the search results but when actually selecting flights give no indication that it is Flexable. The only exception to this is Southwest, which let's you reuse your money on a different flight if there is a cancellation - but appears to be way more expensive than other airlines. Every other booking website and airline seem to make no mention of how to make a trip flexible.

So my question is how do I ensure my flight is "Flexible"? and what does "Flexible" actually entail (i.e. does it vary by airline?, does taking the departing flight allow me to still flex the return flight?, do I pay more if the new ticket is in higher demand or get a refund if it is cheaper?)

EDIT: Ended up going with Southwest because Delta and American were obnoxiously (2x) more expensive, and United had crazy complicated terms for it's flex fares which change for every flight