Hello,
Find the following code for your Slides:
[ch_pre]data-x="['left','left','left','left']" data-hoffset="['100','100','100','100']"
data-y="['bottom','bottom','bottom','bottom']" data-voffset="['0','0','0','0']"[/ch_pre]
And change the data-y value(s) to center.
Please consider checking out the Documentations on the Revolution Sliders completely here: https://www.themepunch.com/revsliderjquery-doc/layer-content/ as these settings have already been explained. If you face any further issues, please feel free to contact us.
Hope this Helps!
Let us know if we can help you with anything else or if you find any further issues.