Variables in functions are local to the function and you don't return the struct; in fact in myUpdate you don't return anything at all...
function myStruct = myUpdate(myStruct)
myStruct.myLength = 2;
end
You would have to use this in some context like
workingStruct=default_config;
... % whatever else
workingStruct=myUpdate(workingStruct);
... % more stuff...
This is going to be an awkward use pattern one imagines and unless there's a lot more than shown going on, simply writing
workingStruct=default_config;
... % whatever else
workingStruct.myLength=newvalue;
... % more stuff...
inline will probably be as legible code and easier...because unless the update value is a constant as shown, you're not providing it in the updating function and if you have to also add it as the second argument, then may as well just go ahead and update the struct itself directly.
Now, if there are some 20 other values as well, then mayhaps some similar structure may be desireable.
