> How can i change a variable ?

You can't.

variables in declarative languages refer to values not memory locations
so asking to change a variable is like asking to change the value of 1.

> But i will change a value of a variable for example when i go to a special 
> template ... and use this variable (with the new value) later in another 
> template with xsl:if.

If the action of a template depends on a variable declare a parameter
for the template and pass the value of the variable in as teh value of
the parameter when the template is called.

David

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