Iqlab (الإِقْلَاب) means conversion; technically it is turning a silent Nun or Tanwin into a hidden Mim before Ba, with Ghunnah. Its one letter is Ba, within a word or across two. Close the lips lightly without pressing, with a Ghunnah of two counts.
Its sign is a small Mim over the Nun, or in place of the second Tanwin mark.
In the Indo-Pak Mushaf its sign is a small Mim written after the Nun, which keeps its sukun (jazm), or after the full Tanwin.
Iqlab
مِنۢ بَعۡدِمِنْۢ بَعْدِ
Al-Baqarah 2:27
أَنۢبِئۡهُماَنْۢبِئْهُمْ
Al-Baqarah 2:33
سَمِيعُۢ بَصِيرٞسَمِيْعٌۢ بَصِيْرٌ
Al-Hajj 22:61
Tanwin before Ba
Common mistake
It is a mistake to press the lips hard in Iqlab, to leave them open so that the Nun appears, or to drop the Ghunnah.