如何从变量中删除这些不需要的级别

Basically ctb2 is city tax band a variable in data set, i just renamed it to make it easier for me, i have produced a summary and a table for the ctb2 as a starter like my advisor says, who also said i need to remove the Us from the variable since they are unknowns so i can just have the levels A to I as they are more relevant to my analysis she also said i could replace them by NAs of which i have no idea how to do that as well been struggling to remove the U from the variable as I have no idea how I need to keep just A to I and get rid of the U and the 952 just standing there. Im quite new to R and been researching on how but nothing make sense. Please help Basically ctb2 is city tax band a variable in d