You can change an existing mask by reopening the Mask Editor and using the same techniques that you used to create the mask:
Select the masked block.
On the Subsystem Block tab, in the Mask group, click Edit Mask.
The Mask Editor reopens, showing the existing mask definition. Change the mask as needed. After you change a mask, click Apply to preserve the changes.
To display a mask dialog box, double-click the block. Alternatively, select the block and on the Block tab, in the Mask group, click Mask Parameters.
Each block has a block parameter dialog box. To view the block parameters dialog box of a masked block, right-click the masked block and select Block Parameters (BlockType).
To see the block diagram under a masked Subsystem block, built-in block, or the model referenced by a masked model block, select the block and on the Subsystem tab, click Look Under Mask.
To remove a mask from a block,
Select the block.
On the Block tab, in the Mask group,click Edit Mask.
The Mask Editor opens and displays the existing mask, for example:
Click Unmask in the lower left corner of the Mask Editor.
The Mask Editor removes the mask from the block.