PiecewiseExpand

PiecewiseExpand[expr] expands nested piecewise functions in expr to give a single piecewise function. 

PiecewiseExpand[expr,assum] expands piecewise functions using assumptions. 

PiecewiseExpand[expr,assum,dom] does the expansion over the domain dom. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.