Update an existing element in a result tree?

Suppose that at some point in the execution of a stylesheet this element is added to the
result tree:

<Member count="1"/>

Later on in the execution of the stylesheet something is encountered that causes it to want to
update that Member element in the result tree so that count is incremented by 1, i.e., the
result tree should be updated so that the Member element now looks like this:

<Member count="2"/>

This may occur many times.  Each time the Member element in the result tree is updated.

Recap: As my stylesheet executes I want it to add things into a result tree, and then as it
executes further I want it to update things that were previously placed in the result tree.
By the time the processor has completed executing the stylesheet the result tree has undergone
all desired additions and updates, and is then serialized.

Would someone please explain how to do this?  /Roger