You don't because the Mosin Nagant M38 doesn't have one. You are probably thinking about the M44, which has a fold over bayonet.
Removing the bayonet from a Mosin Nagant M44 is as simple as removing the screw that holds it on.
See the instruction at SurplusRifle.Com here:
http://www.surplusrifle.com/mosincarbine/rifledisassembly/index.asp
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