How to update an agrument passed by name in scheme

Hi,

Well,  in spite of 30 years of programming imperative languages  ( or
because of that ...)  I can't figure out "How to update an agrument
passed by name in Scheme".

I'm trying to use Scheme to implement an interpreter of a prototype
language,  and it has many advantages,  but this point is beating me
....

Thanks in advance for any help

Ariel Tejera

.