Need a specific character to be changed in the URI

Try this one (remove the logging before applying it in production):

when HTTP_REQUEST { 

    set uri [HTTP::uri]

    log local0. "Uri before replacing: $uri"

    if { $uri starts_with "/global/customers/" } {

        Replacing uri
        set uri [string map {"/global/customers/" "/global/Customers/"} $uri]]

        log local0. "Uri after replacing: $uri"
        HTTP::uri $uri

    }
}

Good luck!

/Patrik