回复：怎样换窗户的外框？

If the rotten section is only a few inches long then you can use a hacksaw to remove the damaged part and cut a piece of wood to fit into the gap. That should be good for a number of years. The important thing is to find the cause of rot (for example, water running from the roof because of clogged gutters, or plants too close to the house that splashes water onto the house when raining, etc).

Get a book from the library to be sure what part you want replace and how to measure. Window case pieces are secured by long nails. Get the right nail removal tool so as not to cause damages to your window.