I have often come across this problem and it becomes so teasing to me. But Now I have found a solution to this problem. Whenever I have typed some words or sentences and I do want to change that word or sentence it does change but will overwrite on other words so previous words will be deleted. This problem is due to mistakingly pressing of Insert Key.
When we press Insert Key it does set the function mode of the keyboard to Overwrite words.
You do need to Press Insert Key again to set the keyboard function mode to Insert words rather than Overwrite words.
The figure below provides the layout of Keyboard showing Insert Key