If you have completed your mandatory 1000-level requirement, you are permitted to take elective courses of any level – so long as any particular course does not have further pre-requisites. However, while choosing your courses, do keep in mind that the levels indicate how advanced a course would be. So although you can take high-level courses, they may be difficult for you.
In addition, do bear in mind, core courses must be completed sequentially. Professors may ask that the 2000 level core courses have been completed by students before opting for their 3000 level classes. In general, it is better to opt for the higher level elective courses after completing the requirements of the initial levels.