These steps are using the Bear Upgrade frame 2.1 as example but can be applied to Original Prusa frames and previous Bear Upgrade frame versions.

Bear Upgrade frame only: Remove the z_end_caps.

Remove the z_tops that hold the Z smooth rods.

Carefully slide the X axis assembly on the Z-axis. Rotate both lead screws to engage the X axis with the trapezoidal nuts.

Continue to rotate both lead screws by hand, to move the X axis assembly down a few centimetres. If the X rods are not parallel with the top of the Z axis, rotate a lead screw on one side only, until the X axis assembly is parallel to the top of the Z axis.

Continue to move the X axis assembly downwards until you reach the middle of the Z axis. Keep the X axis as level as possible.