Alter : To change something or someone ,usually slightly, or to cause the characteristics of something to change.
Synonyms: change, fluctuate, convert, modulate, mutate
Altar : A special table where religious ceremonies are conducted.
Usage of Alter:
1) She wanted to alter her appearance.
2) The service provider would not alter the call charges for about three months from now.
Usage of Altar:
1) He stood at the alter waiting for the priest.