I don't think this is about purifying yourself, so to speak, but more referring to a believer being "purified", post-resurrection. In 3:2 there is reference to "what we will be has not yet appeared"... See scripture below (English Standard Version)

2 Beloved, we are God’s children now, and what we will be has not yet appeared; but we know that when he appears we shall be like him, because we shall see him as he is. 3 And everyone who thus hopes in him purifies himself as he is pure.