To get list of all journals you can use while select statement
LedgerJournalTable LedgerJournalTable;
;
While select LedgerJournalTable
where LedgerJournalTable.JournalName == 'ABC'
{
//Do anything
}
if you want to update existing records of LedgerJournalTable you have to update existing records. for new records change it from journal main form (from where this name is derived)
PS: If you are new to AX, don't worry, just let us know what you want to do. don't share any code, just tell us your business requirement considering us an end user.