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Well, you could look at the help file before asking a basic question , but:
Go to Tools > Script Editor or press F11. Right click on the script at the bottom left called "Main" and choose "New Script." Copy and paste.
F11 -> go to Main bottom left -> press Insert -> Copy-Paste cript
F11 -> go to Main bottom left -> Copy the script from another project (So the whole script, with name and it. Not just the code)
-> Paste it above main.