How do you convert a 6 inch can light into a 3 inch can light fixture?

Recessed Lighting Fixture Cans are manufactured to accept certain types of trim for the type of Recessed Rough in "Can". Check with the manufacturer of the recessed "can" to determine if the rough in "can" will accept the desired 3" light fixture.

If not, then the existing 6" Recessed Lighting Fixture "Cans" will need to be removed and replaced with the 3" can in order to accept the desired trim.

If the "cans" have been covered by the ceiling installed making them difficult to remove and replace the existing 6"cans and install the proper 3"cans, then you can purchase what is called old work or renovation Recessed Lighting Fixture cans and install them in place of the 6" cans. Some work to repair the ceiling would then be required.