There are a couple ways to do this with `:s`, depending on how much typing you want to do. The easiest way is a simple search and replace:

 :%s/find this line/replace with this/
The `%` will search the whole file and replace all occurrences. 

If that's too much typing for you, you can yank the destination string (`yy`) and use the `"` (default) register, inserting it with <kbd>Ctrl+r</kbd> <kbd>"</kbd>:

 :%s/<ctrl+r><">/replacement/
Or, if you have a copy of the replacement string, you can yank the two into separate registers with `"ayy` and `"byy` to yank into the `a` and `b` registers, respectively (you can use any letter for your registers). Then use <kbd>Ctrl+r</kbd> <kbd>a</kbd> and <kbd>Ctrl+r</kbd> <kbd>b</kbd> to paste them into the search and replace fields.