If you can open a .cdb file with Excel editor, it is a good start!
This is how I usually do until now, so there can be better and less complicated ways to edit:
1) When you have open a database (take the DB, you will convert from), it will start to load. Wait until it has ended.
2) It will now ask you for which one you will open up (like dyn_cyclist etc.). I will recommend to open one or two as if you open everyone, it will take time and it will be unnecessarily complicated.
3) I will take example in dyn_team. It will open it on a new sheet. You can now see every rider and their stats and so on.
4) Copy every line except the first line. (You can use Ctrl + Shift, tap shift at the first one and tap again at the last one you will copy)
5) Open a empty Excel file.
6) Paste what you have copied over to the empty one
7) Then close the editor and open it again, where you open the DB you want to convert to.
8) Open dyn_team and delete every teams
9) Then paste from the other Excel file where you copied from the earlier DB.
10) You're done! Look after however the jerseys as they also need to be in PCM 15 folders to be seen ingame.
But it is just my way to do it, and as I self am a novice, there may be some other who have better tips.
Don't forget that from pcm14 DBS to pcm 15 some structures changes. Like some columns are different, others wont appear anymore and stuff like that. You need to pay attention to this as if you copy everything it will crash. Pay attention to which are the same and aren't.
Also before exporting you need to copy the cyclists table, the teams, sponsors and sponsors regions. This is the basic to make a db work. Otherwise it'll crash and won't let you export at the beginning.