How to remove this egr bolt (pic included)

[Pudzey7]

Completely rounded off with 12mm socket . Tried 11mm it didnt work....

[oldcodger]

Several applications of a penetrating spray and the use of a nut remover, like this set, but do check that it will suit the size of your nut and there is the space to attach it to the rounded nut.

https://www.screwfix.com/p/erbauer-bolt-nut-remover-set-6-pcs/6083D?tc=BB3&ds_kid=92700048793290430&ds_rl=1244066&gclid=EAIaIQobChMIoP385cKN6gIVSbTtCh1hVwulEAQYAiABEgIKY_D_BwE&gclsrc=aw.ds

If that fails you will have to cut them off with an angle grinder or hacksaw, assuming you can get behind it to fit new bolts ?