As a rule, the repeat examinations must be taken no later than the examination dates of the semester after next.
If you did not pass an examination in the summer semester 2019, you can repeat it voluntarily in the winter semester 2019/2020; however, you must make the obligatory retry (mandatory attempt) no later than in the semester after next (= summer semester 2020).
You can also enquire about this at the Examination Office and/or refer to the examination regulations applicable to your degree program.
Yes, in some degree programs it is possible to repeat a passed examination in order to improve grades.
However, this is only possible if the exam passed was taken in the semester specified in the curriculum or if the exam was passed in the first attempt. The grade improvement must then be performed by the next examination date available.
You can find out which regulations apply to your degree program from the Examination Office and/or from the examination regulations applicable to your degree program.