Select File > Options > Customize Ribbon.
Click the Customize... button near the bottom, next to 'Keyboard shortcuts'.
Scroll down the list of Categories and select Macros.
If your macro is in the current document, select the document from the 'Save changes in' drop down.
Select the macro in the list of Macros.
Click in the 'Press new shortcut key' box.
Press the desired keyboard shortcut.
Word will show if it is already in use in the 'Current keys' box.
Click Assign to assign the new shortcut.