UPDATE jdtestbysentence."sparseSupplement" SET uuid = 2b22da9c-58a6-11e8-ae82-2d3e941502e8 WHERE a_uid = "1849" IF EXISTS
Ok, so I created your table on my local like this:
CREATE TABLE "sparseSupplement" (uuid UUID, a_uid TEXT PRIMARY KEY);
I ran your CQL UPDATE, and sure enough I got the same error message. Essentially, there is some confusion around the use of quotes here. Double quotes are only to be used when enforcing case on a table or column name. When setting or checking the value of a TEXT (a_uid) you should use single quotes around 1849:
cassdba@cqlsh:stackoverflow> UPDATE "sparseSupplement" SET uuid = 2b22da9c-58a6-11e8-ae82-2d3e941502e8 WHERE a_uid = '1849' IF EXISTS; [applied] ----------- False
Pro-tip: Also, I would caution you against using double-quotes like that. Unless you absolutely need it to match case to a Java class, it's just going to make it more difficult to work with that table. Kind of like it did here.