How is the grammar to increase a stage similar to setstage but instead than a specific stage number set an increment by 10?

Are you after something like:

SomeQuest.SetStage(SomeQuest.GetStage() + 10)

(I didn't test this, but is that what you're after?)

probalby -... Like if I am in quest stage 10 .... when the player does one action , it increases the stage by 10 bringing to 20 then when player does something else it brings to 30 and so on ...

I used GetOwningQuest().SetStage(GetOwningQuest().GetStage() + 10)

Okay, try that then.