how do I add link script to gameobject that already exist with attached script

adding another script to an object doesn't make sense. You can just call the script or it's functions by:

 ScriptName other1;
 
 Start() {
       other1 = GetComponent<ScriptName>();
 }
 
 Update() {
       if (other1.goldCoins == 17) { other1.goldCoins = 99; }
 }
 //short for ScriptName.GetComponent<ScriptName>().goldCoins;

Unless you're creating a new object, and need to add the script, then:

 createdGameObject.AddComponent<ScriptName>();