Updating the Source_Capabilities will do the required job of renegotiating the voltage. Please find the details for updating the source_capabilities in page 86 of CCGx_FW_UserGuide (attached is the document for your reference).
CCGx_FW_UserGuide.pdf 3.3 MB
The Typical process to renegotiate the voltage:
1) GET_SINK_CAP by send handle_pd_command (DPM_CMD_GET_SNK_CAP, NULL, NULL);
2) Enable SOURCE_CAP by source cap mask register by send (dpm_update_src_cap_mask(port, mask))
3) Resend SOURCE_CAP by send handle_pd_command (DPM_CMD_SRC_CAP_CHNG, NULL, NULL);