There are organizations like NEMA and NIWA that are created by the federal government to help flood victims. Private organization or people can also help. There are charities assisting. It can be a help from the government of another country.

https://thenationonlineng.net/fed-govt-plans-special-fund-for-flood-victims/
https://www.thecable.ng/nema-distributes-relief-items-to-victims-of-2022-flood-in-bauchi/amp
https://punchng.com/557-anambra-flood-victims-get-niwas-relief-materials/?amp
https://www.premiumtimesng.com/news/top-news/560651-us-provides-1-million-support-for-flood-victims-in-nigeria.html?tztc=1
https://ng.usembassy.gov/united-states-commits-5-million-in-additional-humanitarian-assistance-for-nigeria-flood-response-efforts/
https://reliefweb.int/report/nigeria/federal-government-supports-victims-flood-natural-and-human-induced-disasters-zamfara-state
https://borgenproject.org/flood-victims-in-nigeria/