Changing Text Box To Input And Ouput In A WebPage?

I created a web page having textbox and an update button. My need is to display data in the text box when page is loaded. After that when I click on the textbox it will become an input textbox and can enter data. when data entered completely, click on the update button, after that newly entered data must be displayed on the text box. i.e same text box will be used for input and output data.

I created my web page using html. I knew that, by using ajax i can implement the above functionality. but i didn't get any idea to write the code.