If you mean change the word "seconds" to the word "cycles", then you can use
Edit/Replace from the menu bar to do that (select a range of cells that
includes the cells you are interested in first); although you will want to
probably press the "Options>>" button to display all the options available
to you (assuming that they are not already displayed) as it sounds like you
may want to make sure the "Match entire cell contents" check box is not
selected so you can find the word "seconds" within other text in the cells.
If that is not what you meant, then you will have to tell us what "cycles"
are you talking about?