The Bible is very clear about baptism. There are two points we all need to understand. (1) Baptism is to take place after a person has received Jesus Christ as Savior, trusting in Him alone for salvation. (2) Baptism is to be by immersion. The word baptize literally means to "immerse / submerge in water." Baptism by immersion is the only method of baptism that adequately illustrates what baptism symbolizes—believers dying, being buried with Christ, and being raised to newness of life (Romans 6:3-4).
With those two key points in mind, what about those who were baptized unbiblically? For the sake of clarification, let’s divide this into two categories as well. First, in the instance of someone who was baptized before he/she became a Christian. Common examples of this are those who were baptized as infants, or those who were baptized later in life, but did not truly know Jesus as Savior when they were baptized. In these instances, yes, such a person definitely needs to be rebaptized. Again, the Bible states that baptism is post-salvation. The symbolism of baptism is lost if a person has not truly experienced salvation by faith in Jesus Christ.
Second, in the instance of those who were baptized after faith in Christ, but in a method other than immersion. This issue is a little more difficult. It would seem to come down to the fact that such a person did not truly receive baptism. If the method was sprinkling or pouring, it does not fit the definition of baptism. Again, the word baptize means "to submerge in water." However, the Bible nowhere specifically addresses those who have been baptized but not immersed. This issue, then, is a matter of a believer’s personal relationship with God. A believer who has been baptized unbiblically should ask the Lord for wisdom (James 1:5). If the believer’s conscience is unsure, it would be best to go ahead and be rebaptized biblically to put the conscience at ease (Romans 14:23).