For starters, you should detach the broken piece from the robot, or to find the missing piece you want to model / modify (on the internet).

Take the broken piece and start to model it, with the measurements from the calliper. Make a 3D model as precise as possible, with all the details you think that are important from the piece. Alternatively, you can modify the part from the internet to be stronger, thicker or things like that.

When printing, high infill is recomended, because the robot parts will be pushed to their limits during the contest.

Before assembling, check that every hole in the model matches with the holes in the robot / the holes in the broken part, both as position and as the size. We have to drill every hole again because our printer makes the holes smaller than it should.

Print some more replacement parts, especially if the broken part was made from aluminium or other metals. Then you can enjoy your repaired robot