Changing of the ن and تنوين
إقلاب means to switch or change something.
In tajweed it is the changing of noon saakinah or the tanween into a م, when followed by a ب with a lengthened ghunnah on the letter ن.It can occur within the same word, or between two words.
It does not change the spelling of the word, rather just the pronunciation, sometimes it is indicated in the text with a little م on the top of the word.
There are two recorded ways of execution:
a) one is where you change the ن into a (sealing your lips, i.e. lips joined during recitation) م and
b) the other way is by leaving a little space between your lips. Both have been recorded by the scholars.
Examples: أَنْبِئْهُمْ, مِنْ بَعْدِ , زَوْجٍ بَهِيج , لنسفعاً بالناصية
(I can’t help but post the following dua, SubhanAllah it so fits with the subject and the words. Truly Allah Subhanah wa Ta’ala controls our hearts and changes/turns them upon this deen, for whomever He, subhanah wa Ta’ala, wishes to guide.)
يَا مُقَلَّبَ الْْقُلُوْبِ ثَبِّتْ قَلْبِىْ عَلى دِيْنِكَ
Ya Muqqalib al Qulūub, thabbit qalbi ‘ala deenak
(O Converter of hearts! Make my heart steadfast in the deen)
Sources: The above notes are taken from an online halaqah on tajweed (which started recently), and the teacher is using the book “بعض المفيد في علم التجويد“, and few websites.