SSH into the instance and root into the database using

use mysql;

Select the user and password column from user table:
If the password column is not found, use authentication_string column name instead.

select user, authentication_string from user;

The output from this query will look like this:

+---------------+-------------------------------------------+
| user          | authentication_string                     |
+---------------+-------------------------------------------+
| root          | *BXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX3 |
| mysql.session | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.sys     | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| redmine       | *2XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXD |
+---------------+-------------------------------------------+
4 rows in set (0.00 sec)

Reset password by filling in the content within the curly bracket:

UPDATE mysql.user SET Password=PASSWORD('{password}') WHERE User='{username}';

Then do a flush to apply changes:

FLUSH PRIVILEGES;