Great question! I know there are times when the same account has been created a second time in error or you're just cleaning up your Chart of Accounts to make things more manageable in your books. In this case, merging accounts is a good way to do that. I see you've mentioned QuickBooks Desktop Pro 2019 in your question although you've tagged QuickBooks Online. I'll outline how to do this in QuickBooks Desktop.
It's a fairly simple process that only takes a few steps.
Select Accountant menu, then choose Chart of Accounts.
Take note or copy the name of the account that you would like to keep.
Right-click the name of the account that you would like to merge, then select Edit Account.
Replace the account name with the one you took note of or copied, then select Save & Close.
A prompt will appear, select Yes to merge the account. Note: If you receive a prompt saying "You cannot merge an account that has online information associated with it into another account" This means you have bank feeds activated that needs to be deactivated.