You stick to one version.
The difference is that Selebus releases updates to his game twice a month (aside from rare exceptions). The first update of the month is titled 0.xx.0 Part 1. The second update, with completely new content, is titled 0.xx.0 Part 2.
0.xx.0 Part 2 has ALL the content of 0.xx.0 Part 1 PLUS about 3 hours of extra content.
So if you have access to 0.26.0 Part 1, rest assured that your game has more content than 0.25.0 Part 2 because 0.26.0 is greater than 0.25.0.
Hope I didn't confuse you!