To add custom php limits like memory limit,Upload limit You can do it from .user.ini file
Create a .user.ini file under your public_html and mention the code
memory_limit=256M;
upload_max_filesize=24M;
post_max_size=32M;
You can edit the value as per your need.If you are using wordpress you need to add the file inside the wp-admin folder too.
Regards
