How can I encrypt my partition?

Hi. (I’m minimalist.)
I encypted my Linux partition once and now I want to put the password away. When I install OS 13.2, I cannot delete this partition, because “it’s being in use”. I even cannot delete the root, home or swap partition!
Is there any way for editing the partitions without formating (ger: Formatierung) and maybe not loosing the stored data on the changed part of the hard drive?

Thanks in advance.