How can my tummy be fixed after I got necrosis? (Photo)

Normal skin from the edges of the area that was skin grafted needs to be advanced slowly over the area of necrosis.  This will take several surgeries to close safely.  How many depends on how tight or loose the skin is at this time.  See a board certified plastic surgeon and good luck.  

Thank you for your question.
Looking at your photo it seems to me that you might have had a skin graft. You have quite a large area in the centre of your tummy of a different colour, texture, thickness and contour. This can be serially excised through several operations 4-6 months apart letting the adjacent skin of your tummy stretch in order to allow the edges of your normal skin get close together until the whole skin graft is eventually removed. As the area is relatively large, you might be looking at several years (e.g. 2 years) for this defect to be completely removed and replaced with a linear scar. I would recommend waiting until the scar maturation process from your original surgery (Apr 2018) is completed (this takes about a year) before embarking on any further surgery. 
Hope this answers your question.