As a rule, the repeat examinations must be taken no later than the examination dates of the semester after next.

Example:
If you did not pass an examination in the Winter semester 2023/2024, you can repeat it voluntarily in the summer semester 2024; however, you must make the obligatory retry (mandatory attempt) no later than in the semester after next = winter semester 2024/2025.

You can also enquire about this at the Examination Office and/or refer to the (subject) examination regulations applicable to your degree program.

If your degree program has subject examination regulations, the General Regulations for Examinations in the Degree Programs at Trier University of Applied Sciences (A-PO, link to the publicus with the files of the A-PO, version dated 04.05.2023, amendment dated 24.01.2024, reading version) apply to you. Here you will find it in § 14 para. 2. If there is a different regulation that specifies a different mandatory date for repeat examinations, you will find this in the subject examination regulations for your degree program. If there is no other regulation there, the regulation in § 14 Para. 2 of the A-PO applies.

Yes, in the degree programs at the UCB it is possible to repeat a passed examination in order to improve grades.
However, this is only possible if the exam was passed in the first attempt. The grade improvement must then be performed by the next examination date available.

For details please ask the Examination Office and/or read the (subject) examination regulations applicable to your degree program.