3d design – How can I move/edit 3D printed holes on an existing STL file in Tinkercad?

The creator has not included anything other than the STL files. This effectively “bakes” the design as presented.

To accomplish your objective, after importing the STL, you could create another solid to fill in the existing holes, then create appropriately placed cylinders and perform a “hole” feature from within the program. The solid does not need to be specifically hole shaped, nor carefully placed, as the purpose is to replace “empty air” with a solid, using the group feature.

If you decide to use this method, note the thickness of the block at the hole location and use that dimension to size the filler block. Use also the plane feature within Tinkercad. This feature allows you to set the zero point and the orientation of a reference plane.

You would set the reference plane to the surface containing the hole to be filled, then move your filler block to a z-zero (new z) over the hole. When you perform the group feature, it should meld into a contiguous block.

The same reference plane can be used to position and locate the new holes. If they are to be centered, the alignment feature will be valuable for this task.