Your default Gmail account is associated with several sites and services, including the default page on YouTube, calendar events, and more. To change it, you need to log out of all existing accounts and then log in again in a browser that saves your preferences. You will now be able to add other accounts to the new default profile.
Steps
Part 1 of 2: Change the Default Account on Gmail

Step 1. Open your Gmail inbox
Before proceeding, make sure this is your default account.

Step 2. Click on your profile picture
It is located at the top right.

Step 3. Click on "Exit" in the drop-down menu
This will log you out of Gmail and all connected accounts.

Step 4. Click on the default account you prefer

Step 5. Enter your password

Step 6. Click on "Next"
You will then be logged into your preferred default account. You can then add other accounts to the default profile.
Part 2 of 2: Adding other Accounts

Step 1. Click on your profile picture

Step 2. Click on "Add Account" in the drop-down menu

Step 3. Click on the name of the account you want to add
Alternatively, you can do this by clicking on "Add an account" at the bottom of the page.

Step 4. Enter your account password
If you add an account that has not previously been linked, you will also need to indicate the email address.

Step 5. Once you have entered all the data, click on "Next"
At this point the secondary account will have been opened and linked to the default one!