If you are the writer of a song, you can change the words however you wish, without asking anyone else's permission. Anyone who is covering your song already will be singing a different version, but that doesn't matter.

If you are singing someone else's song you owe it to the writer not to mess around with the lyrics without their permission, if it is possible to get this. If the writer is uncontactable or dead, then you should explain any changes you make, to your audience, and not attribute them to the original author. Likewise if you change a traditional song (like that old Devon song White Hare - chortle!) you should tell people what you've changed.