I want to remove solder mask in particular area for mount a TO220 package.
As shown in below image I want to mount the 7805 using a M3 nut and need to remove solder mask only in covering area. (like a thermal pad)
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Create a rectangle on the solder mask layer, which is a "negative" layer - anything you create there will not have soldermask.
Because the layer is negative and because the program stacks features, you can do this on the board, if it is application specific, rather than in the library part's footprint.
The advantage of doing it in the footprint (first copy or export the part to a custom library) is that you may have more ready access to the part geometry when defining it, and something that is a common need can stay with the part as a footprint variation.
Conversely, the advantage of doing it on the board is that what exactly you need may depend on the particular board you are designing.
Put a polygon named GND under the tab so it is grounded, it will also help with cooling.