i send a coin to B. In the description field i put in B.
Now B is a label (description) in the history tab but the change address is not labeled with B. Is this right? Do i have to label my change manual?
The description is used to indicate the purpose of the transaction to provide better clarity for the user. It is not associated with any addresses and thus does not affect the labeling of your own addresses.
If you want, you have to label your addresses manually. Note that the labels are only visible to you and local to your wallet file. Unless you have your wallet files, labels or description won't be covered by seed backups.