How to substract a chromatogram that is the first signal instead of last?

I see this option below but my buffer run is my first run and not my last. How can I select something other than the last signal to subtract out?

1. 'Data Analysis'

2. 'Signals'

3. Load the signals

4. Choose 'Subtract the last signal from all other signals'

Thank you, 

Samantha

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