The method of using phpMyAdmin should always work. If you are sure you properly set your password this way and you still cannot login, there is something fundamentally wrong with your WP installation.
Try (via FTP or the cPanel file manager) renaming the /wp-content/plugins/ folder so that WP will deactivate any active plugins, which will eliminate any potential plugin incompatibilities. Try logging in again. If no luck, also go to the themes folder and rename your current theme folder so WP will choose the latest twenty* theme you have. If you don’t know which theme is active, rename the themes folder and create a new, empty themes folder in /wp-content/. Copy the latest twenty* folder from the renamed themes to the new themes. (the folder will probably actually be moved, not copied. This is OK, you’ll just need to move it back after all of this.)
Test login again. If still no joy, go to phpMyAdmin again and search the options table for the “_site_transient_update_core” option. Open the record for editing, but do not change anything and do not click go. Examine the links listed in the option_value field. They contain your WP version as part of the filename. There’s also a s:7:”current” part of the value after which another s:3 or s:5 occurs, followed by your WP version. Remember this version number.
Go to the downloads page and download the version equal to your current version. Extract the files and use them to preform a manual update.
