Hi DG,
Yes unscrew the pommel being careful, make sure to keep the grip, ferrule and crossguard facing the same way when you reassemble the dagger. Again be careful when screwing the pommel back on as to not strip the threads in the pommel, you will feel if it is binding don't force it take it slow.
If the pommel will not turn under light to moderate tension then leave it alone it was probably like that since the original owner.

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Eric