Check that the filter settings is set to 'all stories' and then click ok.
Then enter the text you want to find under 'Find text contains'
Next enter the text you want to 'replace all with'. Then click on start search. It looks for all instances of the text you are looking for and lists them. If you are not sure which to keep and which to change select it and it will be highlighted blue. Then hit the + or - to see which instance it is.
Then with those selected that you wish to change click on replace. Click OK.