How do you take the bayonet of an m38 mosin nagant?

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