If the information you want (Pictures, Movies, Music, Calendars, etc.) is only in 1 account, simply delete the other account.
All the data I want is in user account "Bob".
I also have user account "Bob old"
- Go to Apple > Log out.
- Login to "Bob"
- Go to Apple > System Preferences > Users and Groups
- Unlock padlock at the bottom
- Click on "Bob old" on the left hand side.
- Click on "-" sign at the bottom and choose the last option "Delete"
Now the 2nd account is deleted.
If you have information in both accounts you will have to manually move that information from the account you are you going to delete to the account you are going to keep. This can be done by dragging desired files (Like the iPhoto Library and iTunes folders) into Macintosh HD > Users > Shared. Once in the shared folder you can move the files to the account you are going to keep.
Hope that helps.
OS X will not allow you to delete your admin account if another account is not also an Admin account. As Pondini suggests, make the account you are going to keep an admin before you delete the extra account. If the account was an admin account on the prior system it should already be an admin account on this system after migration.