ovs-ofctl mod-group <bridge> group_id=<id>,type=<type>,bucket=<actions>

Modify a group if the group entry with the specified group identifier already resides in the group table. If the group identifier does not exist, the group will add in group table successfully.
Example:

admin@PicOS-OVS$ovs-ofctl mod-group br0 group_id=100,type=indirect,bucket=mod_dl_src=00:00:00:99:11:11,mod_dl_dst=00:00:00:99:22:22,output:2