Alter and altar can be easily confused because of their one-letter spelling difference. Usually writers know which meaning they want to convey, but they can’t remember which spelling goes with which word.
Alter (with an “e”) is to change or make something different. Altar (with an “a”) has the religious meaning of a place of sacrifice or center of worship.
Here’s a tip for remembering the difference between the two:
Alter is an action, so it requires effort; effort is a word that starts with the letter “e”.