MyBB Community Forums

Maximum File Size (Kilobytes)
The maximum file size for uploads of this attachment type in Kilobytes (1 MB = 1024 KB)

Please ensure the maximum file size is below the smallest of the following PHP limits:
Upload Max File Size: 12M
Max Post Size: 8M

how can i change its limit?

if you want to increase php file attachment size in your forum then you can do at below location
admin panel --> configuration --> settings --> Attachment Types --> check for .php in Extension column -->
click on Edit for .php --> Maximum File Size (Kilobytes) <-- enter required size in kilobytes

in general, maximum attachment size might be set by webhost
AND in case your settings do not work then contact webhost

maximum post size setting available at below location
admin panel --> configuration --> settings --> Posting --> Maximum Message Length <-- enter required size in bytes