Your 6 speed cluster is a freewheel, not a cassette.

If you need lower gears than you presently have, the most cost effective solution might be to replace the freewheel with one that has lower gearing. Much depends on your present rear derailleur and whether it could handle the change. For ex., if your present 6 speed cluster is 14-28, you might be able to run a 14-30, but not much more if at all.

This is the paradox of cheaper folding bikes. Often equipped with low-rent parts that don't lend themselves well to upgrading. The choice of a 6 speed freewheel is evidence of cost-saving. Better folders have cassettes and wider-range gearing.