It is possible to go abroad during both bachelor and master degree studies.
For bachelor students the fifth or sixth semester of studies is a good time to go abroad. In this case you would need to apply at the beginning of your third semester (if you started your degree in the winter semester). It is usually easier to choose the right partner university after you have already completed a few semesters of study in Paderborn. Your chances of being offered a place at the more competitive universities also improve as you will be able to provide more evidence of your academic achievements.
Master students generally choose to go abroad in their third semester.
If you are not sure, it is best to discuss the options with the academic advisor in your faculty as early as possible. Please bear in mind that it is only possible to apply for an exchange semester at a partner university once a year. You must apply at the beginning of the winter semester.