How to change the writing mode in the response box?

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Hi dear mylotters, I got a problem and it really irritated me a lot. I used a old computer to do some responses in mylot before, and a few days ago I bought a new computer then I get a problem with the responses in mylot,and that is in my new computer,when I am writing in the response box I cannot revise my text,if I revise then some letters will missing. And this phenomenon is a little similar with the insert mode and rewrite mode in Microsoft office, in the rewrite mode when you revise your text then you will miss some letters,so how can I change the rewrite mode into insert mode in mylot response box? Looking forward to your advices and thanks for you guys help in advance.