Open your SRB2 launcher. Under "Addons," click "Add" and select the v6 file.
Because "V6" is frequently in a state of active development or limited release, players should look in the following official locations to avoid malware: srb2 modern sonic v6 download top
You can download the most recent stable version from the official : Modern Sonic V5.12 Download (via SRB2 MB). Key Features in v5.12: Open your SRB2 launcher