How can I open a file in version 5.10?
Best Regards,
NaOliveira.

You have to use the neutral file interface and change the subtitle as follows:

#$ VERSION
5.00000 5.20000

TO

#$ VERSION
5.00000 5.10000

Also change:

Data generated by Caesar II / Neutral file interface ver 5.20, 6/07.

TO

Data generated by Caesar II / Neutral file interface ver 5.10, 6/07.

In general, input files are not "downward compatible", this is clearly stated in the Distribution Letter that accompanies each release. The reason for this is that newer versions typcially contain new data fields that earlier versions did not. So attempting to open a newer input file in an older version would lead to a crash.

If you use the [Search] feature (to the right above the calendar), and search for "compatible" you will find a number of posts discussing this subject.