Were the other chapters done in Sigil too?
If so just delete the offending chapter and add file for the new chapter. Make sure it is in the right place in the book browser. Drag and drop it into the right position. If it uses a stylesheet be sure that you highlight your new file and right click for link stylesheet. Make sure the file name fits with whatever other names you have used.
If you are making this for the newer Kindle, I think you should generate table of contents so that it provides the right information. But somebody who is a Kindle person could advise you better on that.