Cheap Chinese linear rails are often coated with a thick grease that prevents rust and corrosion. However this prevents the carriage from traveling smoothly.

Do not use any magnetic tools on the ball bearings. This will magnetize the balls causing more friction.

Remove the 2 screws and the rubber seal on both sides of the carriage block.

Using a small flat head screwdriver to nudge the retaining wire out if it's groove.

The retaining wire needs to bent slightly in order to remove it. Take care to not bend it too much.

At this point the ball bearings are free to fall out so when you remove the retaining wire have a container to put the bearings in ready.

There should be 66 ball bearings.