Xcode offers a handy way to highlight and edit all references to a variable within the current scope. Here are the steps:
First, make sure you have the setting shown below enabled within the Text Editing tab in the Xcode Preferences:
Double tap a variable name and keep the cursor over the marked text. A drop down arrow will appear to the right of the variable.
Selecting the dropdown arrow will show a list of options, choose Edit All in Scope:
Xcode will highlight all references to the variable name:
At this point you can edit the variable name and all changes will cascade through each variable reference: