To change a numeric parameter like the measurement bandwidth, select this value from the 'SETTINGS' or 'SETUP' menu. The upper display line describes the parameter, the lower line shows it's value:
To set a new value, press 
. This changes to the editing mode, signalled by a '>' character in the first column. Use the 
keys to change the value.
To accept the changed value, press . Pressing 
leaves the editing mode without changing the value.