Manually update links
 

I have an Excel document that links to several others. I want some links to
update automatically but some to be manual. (I will still have the prompt to
update links on opening).

I don't have the option in Edit Links to select Manual - it is greyed out.
I have tried various combinations of Update Remote References and Save
External Link Values with Calculation Manual and Automatic.

What am I doing wrong? I do not know where else to look.