They are not mutable, you cannot change each component independently. Actually you can’t have a mutating function (indicated there by the !) written for StaticArrays. The “best” you can do is to redefine them and return the new ones.
So, as a rule of thumb, none of the mutating functions are expected to work on static arrays (although they may provide that possiblity if they return the array as well, depending on what the function does).