In the currently available public information, there is no confirmed way to get a double jump in Hollow Knight: Silksong
The game has not had its full release details publicly confirmed for all abilities and progression
If you are asking about the original Hollow Knight, the double jump is the Monarch Wings ability
In Hollow Knight, Monarch Wings are obtained in the Ancient Basin after progressing far enough to reach the relevant area
For Silksong, check the latest official updates, release notes, or in-game progression once the game is available