I have a 70 amp breaker for a hot tub i'm getting rid of the new one is only 50amp can I use the same breaker?
If the manufacturer of the hot tub recommends a maximum circuit size of 50 amps then you should change the breaker to 50 amps. The conductors can stay the same. 

If the 50 amps is a minimum rating for the hot tub then you can leave the 70 amp breaker and conductors.

You should always follow the manufacturer's instructions. The National Electrical Code requires it.

Good luck!