JSFiddle - React, Tailwind, and code Playground

JavaScript

function ReplaceAt(input, search, replace, start, end) {
		return input.slice(0, start)
    	+ input.slice(start, end).replace(search, replace)
      + input.slice(end);
	}
  
alert(ReplaceAt("my first my test", "my", "uuu", 9, 11));