If someone have missed many Salahs in their account. Either one time's or of many years, they must pray their Qaza as soon as possible. Salah is a Fardh and not forgiven but must be made up. On the day of judgement Salah will be the first thing asked about.

For the people who have many years of missed Salahs. There is a way to pray them quickly. The following instructions have four exceptions and has all Fardhs and Wajibs that complete the prayer. Please pray your Qaza as soon as possible. Even of you could pray one days Qaza Salahs every day which are only 20 rak’ahs (3 Wajib Witr), please do it. It only takes few minutes to perform 20 rak’ahs according to the following instructions.

• In Rukuh and Sajdah instead of reciting "Subhaana Rabbiyal Azeem" and "Subhaana Rabbiyal A'la" three times, say it only once. But make sure do not leave Rukuh posture until the Meem (M) of Azeem has been said properly. Similarly do not leave the posture of Sajdah until Ala has been said completely. Just make sure to say these Tasbeehaat (the recitations) properly and do not rush.
• In the third and forth Rak'ahs of the Fardh Salah instead of reciting the whole Surah Faatiha say, "Subhan Allah" three times and go to Rukuh, Make sure "Subhan Allah" has been recited three times properly, do not rush. This exception is only for the Fardh prayers. In the third rakah of Witr it's a must to recite full Surah Faatiha followed by at least three ayahs of Quran or a Surah (as we usually do in first and second rakah).
• ) In last Qa'dah (when we sit for Attahyyat) before Salam, after Attahyyat instead of reciting the whole Durood Shareef (Salawaat) and Du'a just say, "Allah Humma Salle Ala Sayedena Mohammad Wa Aalihi", then finish the Salah with Salam. Du'a is not a must here.
• In Witr, instead of full Du'a-e-Qunoot just say "Rabbigh Fir Lee" once or three times.
  (Fatawa –e-Rizvia, Part 3, Page 622)