Or you can press 'R'
It works better if you select all you want replaced, and do them all at once. The behaviors and instance variables have to match between the old and new, so you may need to add them first.
Here is a tutorial on replacing an object with it's family, but it explains replacing pretty good.
If not using a family start with the 3rd paragraph under 'The Process'
https://www.scirra.com/tutorials/535/how-to-upgrade-an-object-to-a-family