Help! I've pressed some weird button on the computer! What do I do?

When I'm making adjustments to something I've already typed, instead of just inserting the new letters, it 'eats' the letters I've already typed! (sorry that's the only way I can explain it!)

How do I change it back to normal??
Update: thanks everyone who said to press "insert" it worked - yay! :-)

You've probably guessed I'm not very computer-savvy!
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