You can increse memory limit via .htaccess or php.ini file.
try one if dnt work try other one.
in your root folder create file php.ini and save it. edit the file with code editor and enter this code and save it
memory_limit = 30M
if won't work above then you try to increase via .htaccess file
paste this code below in your htaccess file
php_value memory_limit 30M


replace the 30m with your required size limit.(if you want unlimited then put -1)

make sure that your host allowed increasing limit...
..
if you have WHM then you can easily change limit
go to server configuration> PHP Configuration Editor edit from there
.........
check phpinfo() to indicate the changes after you edited

thanks