Quote from Excalibuurr;535042:
Just to clarify, can he go back to the previous time lines?
more or less possible... This information is based on the theory you watched all episode up to 17.
In episode 9, Ferys Nyan Nyan sended a D-mail to prevent her father from dying, It cause the world line to switch from 0.456903 % to 0.409420 %
in episode 10, Okabe sended a D-mail to follow Suzuha and prevented her from leaving in her time machine. This D-mail cause the world to shift from 0.409420 % to enter World lie 0.337187 %
Up to episode 16, Okabe did various Time leap which changed how his actions were going to be but that wasn't enough to change the world line which stayed 0.337187 %.
At the end of episode 16, he did a D-mail that canceld the previous D-mail so he never followed Suzuha. This caused the world to shift from 0.337187 % to 0.409031 %
It is a slightly different number but it's very close to been the same of episode 10.
In episode 17, Okabe cancle out the D-mail of episode 9 and cause the world line to shift from 0.409031 % to 0.456914 % That new number is very very close to what it was before epsiode 9.