Convert images from 256 to 512 !

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I tried doing it with Fshx , the double resolution method , but after you do that it creates a 256 mipmap ,and then the texture wont even appear !!
Is there a more simple way to convert textures from 256 to 512 ?
Do i delete the 2nd mipmap and if so how ?
Thnx for youre help , its really apricited !!