Method 1: Edit file wp-config.php

Add the following to wp-config.php 

define( 'WP_MEMORY_LIMIT', '256M' );

(usually place under define(WP_DEBUG, true);  like this https://prnt.sc/tk9gp2)

Method 2: Edit file .htaccess

Make sure you back up .htaccess before you edit it. 

Add the following to .htaccess:

php_value memory_limit 128M

Method 3: Edit php.ini

Add the following to php.ini:

memory_limit=128M