Before you can perform this task, you must have regularly-sampled, steady-state time-domain data imported into the System Identification app. See Import Time-Domain Data into the App). For transient data, see How to Detrend Data at the Command Line.
You can use the shortcut Preprocess > Quick start to perform several operations: remove the mean value from each signal, split data into two halves, specify the first half as model estimation data (or Working Data), and specify the second half as model Validation Data.
In the System Identification app, drag the data set you want to detrend to the Working Data rectangle.
Detrend the data.
To remove linear trends, select Preprocess > Remove trends.
To remove mean values from each input and output data signal, select Preprocess > Remove means.