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Only a step which is currently under execution can be modified.

Limitation

You cannot modify the parameters of the steps that are being provided using datasheet. On ly the parameters that re being provide using static data can be modified.   

Modifying Parameters of Step

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Modifying Conditional Structure

You can modify conditional structure using modify step functionality of debugger. This option is available on the context-menu (right-click) of the current execution pointer.

 The test case execution during debugging will get terminated with appropriate notification for an invalid expression.