Depends on region.
This word 辮髮 is quite transparent: 辮 braid + 髮 hair (of the head).
Hence, in standard Taiwan Mandarin, as defined by Ministry of Education there, both characters have a single pronunciation each, and so the word as a whole is pronounced biànfǎ.
On the mainland though, the second one 髮 was defined as being pronounced as fà on its own. This is where 辮髮 is pronounced biànfà; however, I do not see authoritative sources which say this is prescriptively correct, despite it being a common way of pronouncing this on the Mainland.
The character 髮 was merged with 發 fā and underwent character simplification to form 发. As 發/发 fā has become more common, this is where 辮髮 = 辫发 gained its third pronunciation, biànfā, which is the pronunciation on zDic.net.
According to Xinhua Zidian, the only admitted standard pronounciation currently for 发 (in Mainland) is
- fā, various meanings e.g. "to send out, to dispatch", "to start", "to grow, to expand", "to open, to reveal, to invent", "to show", "to produce, to happen"; and
- fà, meaning "hair".
As 发 in 辫发 refers to hair, it should be pronounced biànfà in Mainland China. This is also how I pronounce the word (native speaker).
In Taiwan, as indicated by the other answer, fǎ is correct.