Android: How do I change a value that is defined in the xml within the java code

Android: How do I change a value that is defined in the xml within the
java code

I have multiple tabs with text on them. I have another tab which is
responsible for holding the GUI elements to change the value of the
default text size. I already have a default value set in the xml files.
How might I go about doing this, creating a variable that is accessible to
all activities?