m = 'dec'
y = 2012
l = ['Item_has_was_updated_May_2010', 'Item_updated_Apr_2011',
    'Item_got_updated_Sept_2011']
r = []
for s in l:
    t = s.split('_')
r.append('_'.join(t[:-2]) + '_%s_%s' % (m, y))
Replace items in list, python