Ok, Ideally, you should have had those bigger and or in vector. But as it is, here it goes.. This is in photoshop:
Select the checkmark.
You can do this by using the "select in quick mask mode":
When you have switched modes, select the brush:
Use the brush to "paint" over the checkmark. When you are happy with your painting, that you have covered the pixels you need, switch over to "normal mode" (the first button we switched).
If you are not happy with the selection you can switch between the modes. When you have got all the pixels, copy it. You can make a new document and paste this checkmark in, in case you might need it later.
Paste and place the checkmark in on your other icon: