更改http.Client cookie值

I have the below code that works as intended, however upon the first request, I have the cookies I want, and just want to change a value of 1 cookie before sending another request. So far I have been rather unsuccessful.

jar, err := cookiejar.New(&cookiejar.Options{PublicSuffixList: publicsuffix.List})
if err != nil {
  log.Fatal(err)
}

client = &http.Client{
    Jar: jar,
}

firstRequest() // aka login

mainLinkedinURL := "http://www.example.com/"
cookieURL, _ := url.Parse(mainLinkedinURL)
for j, i := range jar.Cookies(cookieURL) {
    if i.Name == "JSESSIONID" {
        jar.Cookies(cookieURL)[j].Value = "Another New Value"
        i.Value = "Another way of setting a new value"
    }
}
secondRequest() // request after changing cookie

This is only from the http.Client perspective, and no server-side cookie management.

To change value of cookies you need to use SetCookies method.
Because you need just to change a single cookie value you need something like this:

mainLinkedinURL := "http://www.example.com/"
cookieURL, err := url.Parse(mainLinkedinURL)
if err != nil {
    // handle error properly
}
cookies := jar.Cookies(cookieURL)
for _, cookie := range cookies {
    // note: if there is no cookie with such a name 
    // then a new value would not appear, please pay attention to this
    if cookie.Name == "JSESSIONID" {
        cookie.Value = "Another New Value"
        break
    }
}
jar.SetCookies(cookieURL, cookies)