Hello dinistic,   I think this may be what you are looking for.    Change the primary boot parameter to the upgraded partition (A: or B:).   DellEMC(conf)#boot system stack-unit all primary system: b:   Then write memory and reload   Page: 8  - Dell Networking S3100 Series 9.14(2.16) Release Notes [URL]  
Hi good morning. I have several stacks of DELL S3148P with unaligned boot partition.     but the stack-unit 2 keeps booting from A: instead of B:Any thoughts?    
S3100 change boot order of the stack-units