nextel to boost

i have 2 nextel phones 1 i90 and 1 i60 how would i go about changing to boost?

you need to purchase a boost activation kit. The only problem you may have is not all markets have them and if they do they may just have the 3 digit series sims. At the worst you will have to buy the i205 boost phone and activation kit for $80 and keep the sim sell the phone. The dealer might buy it back from you.