Translation

バックルを外す / ベルトを緩める

Meaning

Release or undo a fastening mechanism, typically by pulling a pin or tongue out of a buckle.

Examples

• She had to unbuckle her heavy leather belt to get comfortable.
  She had to unbuckle her heavy leather belt to get comfortable.
• Please unbuckle your harness before you attempt to climb down.
  Please unbuckle your harness before you attempt to climb down.