Several answers here...
As you already had a previous version, are you sure about which database you accessed to (Prefix can be a clue).
Check your database, you may have use a different prefix which could explain the issue. Note this is not the ony source of issue...
If you do want to go from scratch, I imagine you kept the old Chamilo version as it was so you still can revert to previous step (initial environment/configuration).
If so, you then can gently remove the directory of the newer chamilo version (assuming you didn't put it in the same directory as previous revision).
After doing this, your chamilo may accept to reinstall.
An drastic overcome is to :
1- Save your entier databases
2- create a new database (e.g. cham1884)
3- install the new Chamilo version addressing cham1884 (or the name of the new database).
Note that you don't go for an update in such circumstances, but you can then export all your content ad import it in the new version, and do the same with user.
Depending on the database architecture, you may even be able to copy some info from older to newer version.
Last but not least, please, if you have another way to do it, please feel free to share it with us.