Variable re-assignment in config

Is it possible to re-assign a variable in the config file? (eg.
$request_uri)

I’ve tried using set but that yields:

the duplicate “request_uri” variable

And trying to use an = yields

unknown directive “$request_uri”

I’m trying to find in the docs where it talks about variable assignment
but
I’m not finding it.

Thanks for any suggestions on how to do this,

~Wayne