How are levels changed?
What are "levels"?
If you could clarify what you mean by Levels for Duplicates and Sources it would help answer your question. Normally Duplicates all need to be reviewed to see if they are the same person and if they are then they should be merged. Sources offer an opportunity to find more about the individual, always important to view the source if possible as there is often information on the source not indexed.
We would suggest working on duplicates 1st as often the sources are attached to a duplicate person, although at times a source can help verify if the duplicate should be merged or not.
In the help center two searches that may be of help
How to merge duplicate people?
How to attach sources? How to use the source linker?