How to change Wordpress image thumbnail?

Want to create an image gallery into a dynamic page in Wordpress but the images are always cropped to 150x150 px, and I need at least 300x300 .
tried: in settings - media - change the size of thumbnails does not work .
+ there is code in functions.php
if ( function_exists( 'add_theme_support' ) ) {
 add_theme_support( 'post-thumbnails' );
 set_post_thumbnail_size( 500, 500 );

need it to output images of a certain size on the blog page.
In General, do not know what to do , please tell me how to be
June 7th 19 at 14:57
1 answer
June 7th 19 at 14:59
resizing of thumbnails not working

After changing parameters, you should regenerate the thumbnails, for example, using the plugin Regenerate Thumbnails.

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