How to increase Website DA ?

but why do you want to increase the DA? I hope it is not because you think increase DA will increase your ranking. They are just a number and the focus should be to increase the ranking in google
Though I may be wrong if you have plan to sell the domain or accept advertisement (where advertiser pay you based on DA)

I have just 100 backlinks but DA is 31

the option right now is getting a lot of links from high DA sites with the latest DA refresh update.

Get a good domain name
remove bad backlinks
buy good backlinks