You can't do that. It took over 10 years of requests for FileMaker to come up with a script editor that even begins to work like a typical script text editor and such a capability just doesn't exist in the current scripts workspace.
If you have advanced, you can generate a database design report such that you can do a text search for a given field name and thus find each place where it is used in a script, but that's a far cry from "find/replace". You can also go to Manage | Database | Fields and just rename the field--which will cause the new name to appear in each existing script step as long as the field reference is not in the form of a quoted text string.
I have advanced. But doing that in hundreds of scripts is not really an option. I am working on a system (originated in FM4 !). It is well written but has lots of scripts. DDR reports only help to find things but it really is a pain to have to go to each script one by one, try to find the part you want to replace and doing that manually. How can you do serious programming without this? I also found that 'undo' is not an option in scripts. Why they call this 'Advanced' ?
Advanced offers many other tools that both save you a lot of time and allow you to set up solutions with user interfaces that simply cannot be designed with FileMaker Pro.
There are third party tools that can take the DDR produced by Advanced and do the type of thing you need and much more besides. We use Base Elements for that purpose here. It's quite possible to perform a find (In a FileMaker Database of data from the DDR) and find every reference to that one field.
BTW, you ARE aware that you can go into Manage | database | Fields and simply rename a field to see it appear with the new name throughout your solution? Whenever possible, FileMaker manages field references by an internal ID, not the name so name changes don't normally require a "find/replace" action unless you are importing the script from a file that has such a field to a file that does not.
The other exception is when you use indirect, name based field references where the name of the field is enclosed in quotes. In nearly all cases, it's possible to set even these references up so that a change in Manage | Database does not "break" the expression.