Installing a custom ROM on a Xperia M (Nicki) requires the bootloader to be unlocked on the Xperia M (Nicki) phone, which may VOID your warranty and may delete all your data. Before proceeding further you may consider backup all data on the phone. You are the only person doing changes to your phone and I cannot be held responsible for the mistakes done by you.
Read detailed guide his to install LineageOS on Xperia M here
Official Builds by LineageOS Team download links:
Released : 2017-04-27 10:49:06
Released : 2017-03-09 10:42:00
Released : 2017-03-02 07:00:51
Released : 2017-02-22 21:49:11