What happens to different deities we worship at the end of their life cycle. Will they merge back into Parabrahma?
As per Vishnu Puran: Part 1: Chapter 15,
There were twelve celebrated deities in a former Manwantara, called Tushitas, who, upon the approach of the present period, or in the reign of the last Manu, Chákshusha, assembled, and said to one another,
"Come, let us quickly enter into the womb of Adití, that we may be born in the next Manwantara, for thereby we shall again enjoy the rank of gods:"
and accordingly they were born the sons of Kaśyapa, the son of Maríchi, by Adití, the daughter of Daksha; thence named the twelve Ádityas; whose appellations were respectively, Vishńu, Śakra, Áryaman, Dhútí, Twásht́ri, Púshan, Vivaswat, Savitri, Mitra, Varuńa, Anśa, and Bhaga.
These, who in the Chákshusha Manwantara were the gods called Tushitas, were called the twelve Ádityas in the Manwantara of Vaivaśwata.
Below are the Sanskrit verses and Hindi translation:
But, it's not always the case. As mentioned in this post, in 8th Manwantara, the Indra will be Bali, the sinless son of Virochana instead of current Indra (Śakra or Purandara).
So sometimes, deities take rebirth and again attain the post of deities and sometime they don't attain the same post.
Finally, they merge back into Brahman (Maha Vishnu as per Vaishnavism), for Manu it is mentioned in PADMA PURANA: SECTION I: SRSTIKHANDA: Chapter 7 Description of Manu Periods:
Past and future Manus are (thus) described (by me). O prince, a thousandyugas will be occupied by (each one of) these.
Having created all these mobile and immobile beings in their own periods, they will, at the end of the kalpa, obtain release along with Brahma.
At the end of a thousand yugas they will again and again perish, and led by Brahma will then be absorbed in Visnu.