How to change ACL forbidden page?

Then how are you using ACL as this is APM feature ?

https://techdocs.f5.com/kb/en-us/products/big-ip_apm/manuals/product/apm-network-access-13-0-0/3.html

https://community.f5.com/t5/technical-forum/apm-l7-acl-error-page-for-network-access-cannot-be-displayed-on/td-p/47008

For custom responce you may use ACCESS::respond with events ACCESS_ACL_DENIED, ACCESS_POLICY_COMPLETED

https://clouddocs.f5.com/api/irules/ACCESS__respond.html

If you are using just LTM then maybe an irule  or packet is blocking your traffic. For packer filters you can't change the page but for irules:

https://support.f5.com/csp/article/K14702105

Please in the future investigate and formulate your question with more detail if you want proper assistance. If you need some help getting familiar with F5 I suggest the getting started videos and F5 operations guides for different modules (LTM, GTM , APM etc.)