If your function block is implementing an interface and you have added more methods or properties to the interface, you can update your function block to get all the additions from the interface. 

In the image below, we can see that I_Calculate interface has a new method called 'Clear'. FB_Calculate function block is implementing I_Calculate interface. Apparently, 'Clear' method didn't exist when creating the FB_Calculate function block. 

You can right click the function block and select 'Implement interfaces' to update the function block.

 Select the implementation language and click OK.

Now we can see that FB_Calculate function block has the Clear method as well.