"Yesterday" is not whatever you imagine up in your head. It refers to a time which the context is describing. The writer is referring to a past time in the lives of these Christians when they first believed. The Hebrews writer tells them to remember that time and then reminds them that Jesus is still the same since that yesterday when they first believed. Since the beginning of their Christian life, yesterday, the RISEN Jesus has not changed; he is still the same and will always be the same.