To fix Element not foundNow error in Selenium IDE then you can easily update object id rather than updating in the way above:-
Firstly install Firebug for Firefox.
Open the website that you want to inspect.
Click the Firebug button.
Click the element inspector button.
Click the web page element that you want to inspect.
Right-click the highlighted code in the Firebug panel.
Select "Copy XPath" from the menu.
If you want to learn Selenium then you must take up the following Selenium training Course. You can also read the following Selenium tutorial if you like to read things. You can also watch the following video tutorials to learn all the concepts of Selenium.