Please take the time to use the search function at top of page.
You will find at least two threads about that exactly subject. Also there is a tutorial. Even is a 3ds noob thread.
For short you need to put THE NEW card on the 3ds at least one time, then put both cards on your pc, you will find a folder:
"SD:\Nintendo 3DS\<32Hexdigits>" on each card, you need to copy everything from your old card to the new card, but you need to copy the content of the OLD "SD:\Nintendo 3DS\<32Hexdigits>" folder to the "SD:\Nintendo 3DS\<32Hexdigits>" ON THE NEW SD.
By example, you old card have:
SD
|-3ds
|-gm9
|-luma
|-Nintendo 3DS
|--ABCDEF1234567890ABCDEF1234567890
|--private
And the new
SD
|-3ds
|-Nintendo 3DS
|--0987654321FEDCBA0987654321FEDCBA
|--private
You must to copy the contents of SD:\Nintendo 3DS\ABCDEF1234567890ABCDEF1234567890 from the old card to SD:\Nintendo 3DS\0987654321FEDCBA0987654321FEDCBA on the new card.
the others folders on the root can be copied exactly.