How can I get seasonal outfit MILLEN FEDORA (Warrior)

You need to fuse two together, and you only have a “chance” of getting it.

Which two outfits I have to choose can you tell me in details

Any 2 costumes. You combine and have a tiny chance like less than the tiniest chance to get season outfit.

Yep, like others said, any two outfits can be fused (click the "fuse" button under the seasonal preview, or on the outfit itself in your inventory) then pick 2 outfits, it will delete one and give you either the same outfit or maybe the seasonal one, but the seasonal is NOT a guarantee when fusing. You are like 99% likely to get the same outfit you picked to fuse, and NOT get the seasonal.

Which is a goddamn shame. Bern for the Ranger is my absolute favorite outfit in all of BDO, I've had mine since it first was avaible after launch and it's great. But it looks like you need to buy hundreds of dollars worth of pearls in BDM just to rng your way into accidentally getting the outfit. And I like Pearl Abyss, I love BDO and BDM...but this is nonsense. I hope they add the current seasonal outfit as purchasable next "season" and change the seasonal to a new one, or something. It would be super nice if they, you know, told us anything about how that works. 

I wish they have have more outfit collections and 100% success rate on fused materials.... But overalls if we speaking this game just open to global now so it's take a time but if I say I want it every optionable function and new playing characters unlocked too asap..

I just hate that the one thing you can do with an outfit/armor that you can't force to be 100% success is the only thing that you must do with RMT items. Like, if I'm playing for free, I can change appearance of things, swap enchants, etc, all with 100% if I know what I'm doing. When I pay them money, they will give a tiny chance at getting what I want. 

If I could just buy the seasonal outfit, I would. But fusing two outfits, even if we get two for free, for just a tiny chance at a special one? No thanks. 

Which, again, sucks. I love me some Ranger Bern. Hopefully they'll add it as purchasable directly. I know in BDO they never announce when outfits will be on or off sale (presumably to drive up demand). So I'll wait. 

So question if I have the seasonal outfit and I try to fuse it again is there a chance of losing the seasonal?

I receved 1 outfit, not 2.