OK wonky grammar post ahead:
"Alter"--to change. "I altered the dress to fit my sister."
"Altar"--the "table" in a church. The place where the Eucharist is consecrated. Whatever you want to call it. "The servers approached the priest, who was at the altar."
THEY ARE NOT INTERCHANGEABLE. THANK YOU.
PS--I would also like to add that one is a verb, the other a noun. So they are REALLY not interchangeable. You cannot get up and "altar", as my seventh grade English teacher would say. You can, however, get up and "alter" something.