The word alter is a verb, and means to change something a little bit. The noun form of alter is alteration, which means a small change.Another word that comes from alter is alternative, which means another possibility or another option.
Example -
1. Have you altered your hairstyle?
2. I'm not happy with the design of the kitchen. Maybe we should make some alterations.
3. I'm not sure if I agree with your idea. Perhaps we can think of an alternative option.

The word altar is a noun. It has two meanings, both related to religion. Altar can refer to the table at the front of a church where priests perform ceremonies.
Example - The wedding ceremony was so beautiful. The groom looked so happy when he met the bride at the altar.