A Step by step guide to update rom

Hi friends,
I am using mmx a117 and want to upgrade rom.

I have found some roms here in xda, but not completely sure about how to update,
My main aim is to increase internal memory from 1 gb to maximum.
and second aim is to get any update on jellybean like lolipop or Marshmellow

I request you to please guide me step by step to update rom. am new n never did anything recovery mode.

pls guide

thanks