Unfortunately you have to choose between the two. Either you forfeit your seat at ECE NIT Patna and sit for CSAB, as per CSAB Rules and Regulations you cannot keep any seat at hold in JOSAA. Perhaps,  I believe you should now freeze NIT Patna, it is an excellent and old technical institute with good placements as far as ECE is concerned and good faculty. Comapnies like Amazon, Qualcomm, Reliance Jio, Microsoft, Tata Communications, Samsung are regular recruiters at ECE NIT Patna and you can always study in first year and opt for a branch change to CSE. Besides this ECE is a very structured branch, and you can always learn new skills and master new technologies involving Machine Learning and Software.

For more information regarding CSAB do follow the link below

https://engineering.careers360.com/articles/csab