How to override 'proxy_set_header

[Gelonida]

Still new to nginx, and having some basic problems.

I'm having a config file 'proxy.conf'
which I normally include in  locations, which should be proxied.


for one location I'd like to override a header specified in the 
proxy.conf file.


what I tried

location /blabla {
   include.proxy.conf;
    proxy_set_header Host custum_val
}


However this just appends my customval to the already existing 'Host' 
header.

Is there a way to override a header?

I considered changing the variable $host

However:
I don't know the syntax to do this.
I don't know who else uses the variable $host, thus I'm not sure whether 
some side effects would show up.

How do you handle such situations?