How do I get this symbol ~ to appear over a letter?

Answer 1

It depends on which letter you want it over.

for ñ, the most common use - just use alt+164 on your keyboard.

example: as in Niño

If you're on a laptop, you have to use the numbers in your number pad. There is a number pad on your laptop it just may require using the FN or some other keyset.

Answer 2

Save it as a pic, then place it over the letter. You will have to format the picture to be behind the text. Right click on the pic to do this (shortcut).