while finding Vrms or Vavg we use integration and we use d(wt) and integrate between the limits 0 to 2pie. When 'w' is a constant how can we take it as d(wt).and if we take as d(wt) only then why are the limits not changed to 0 to 2(w)(pie)?
Yes you're kind of right. You would change the limits of integration as appropriate. But remember that the original limits of integration would normally be t=0 to t=T (the period). But T = 2 Pi / w therefore wT = 2Pi is new limit after the change of variable.
Mathematically when we make the substitution [itex]\theta = \omega t[/itex] we get: