Are you sure is a dns level ban?
As for changing it, it can't be any simpler than this, at least for Android as that's what most use, just go to settings, type DNS and you get the Private DNS settings, and you change it from automatic or off to custom as you add to Private DNS provider or DNS provider Hostname for example "dns.google" as I doubt the Nigerian authorities have enough power to force google to enter customs bans.
Not yet cause we still have access to exchanges apps but not the website. These exchanges still provide services to Nigerians( we're not restricted). It internal access restrictions by our service providers but I still feel they're not satisfied yet and might go further to issue DNS level ban sooner or later.
The last CBN Governor stated that Bitcoin is created out of thin air and might disappear at any time.... He is about to serve a life sentence in prison for embezzling public funds, while Bitcoin has just reached a new all-time high. Who is disappearing into thin air right now?
The new CBN Governor stated that Binance is the cause of Naira depreciation and got the NGN trading pairs removed from all exchanges, but the Naira continues to depreciate against the dollar.
When the government runs out of options, it blames cryptocurrency, which is almost always wrong. Cryptocurrency is not the enemy.