There is a way but this feels like there must be a better way! However, as you've not had an answer, and you do not explain how much text you need to update in this manner, I'll provide it.
Enter some text in a cell, click in the cell and type the letter a
. Whilst the caret is to the right of this text go to the Insert ribbon and the last icon should be Symbol. Click on it
In the box called subset, choose Combining Diacritical Marks
Choose the symbol you want and it will combine! However, this appears to only work character at a time!
Another option may be using a border and applying it to the top!
Or, if you're showing a maths formula and need the X with the line above, click on Symbol and choose the font MS Reference Sans Serif form within the Symbol dialog. Then type in F7C4 as the unicode number.
Also see https://stackoverflow.com/questions/33283669/write-text-with-overline
Another option is to search for a font which has this effect!
Another option is to use the line tool in insert ribbon and illustrations tab, see Shapes and then lines.
The issue you may have is that I don't believe Excel supports it. If you try to copy HTML and paste into Excel, the formatting persists. It persists for underline and strikeout, but not overline.