Since my brother is a student of the School of Open Learning (University of Delhi), he was able to offer better insight on the topic. Here is what he had to say about the improvement rule mandated by the institute.

1. The only exam you are permitted to improve is the last one that you appeared for. For example, let’s say that you appeared for the B.A 2nd-year examination then it is possible to only improve it once later on. While it is not possible to implement the improvement rule on the exam, like B.A 1st-year, that you appeared in the previous year.
2. Now, it does not necessarily matter if your score either increases or decreases because whatever you score is going to be the ultimate score you receive. The marks that you retrieve will be final and you will have to surrender the previous score when you apply for the improvement exam. 

So, if you want to opt for an improvement, you need to fill the UnderTaking form. Attach your original mark sheet with the form. In case you didn’t get the original mark sheet, you can contact SOL.