ClearAll["Global`*"]
\[CapitalLambda] Λ = -30;
u[\[Eta]_]u[η_] := (2*\[Eta]2*η - 2*\[Eta]^32*η^3 + \[Eta]^4η^4) + \[CapitalLambda]Λ/
6*(\[Eta]η - 3*\[Eta]^23*η^2 + 3*\[Eta]^33*η^3 - \[Eta]^4η^4);
\[Theta]θ = Integrate[u[\[Eta]]*Integrate[u[η]*(1 - u[\[Eta]]u[η]), {\[Eta]η, 0, 1}] // N;
\[Delta]δ = 1/\[Theta];θ;
u[y_] := Piecewise[{{1,
y > \[Delta]δ}, {(2*y/\[Delta]δ -
2*(y/\[Delta]δ)^3 + (y/\[Delta]δ)^4) + \[CapitalLambda]Λ/
6*((y/\[Delta]δ) - 3*(y/\[Delta]δ)^2 +
3*(y/\[Delta]δ)^3 - (y/\[Delta]δ)^4), True}}]
Assuming[yd \[Element]∈ Reals,
FindRoot[Integrate[u[y], {y, 0, yd}], {yd, 1}]]
{yd -> 6.20224*10^-9}
OR
\[CapitalLambda]Λ = -30;
u[\[Eta]_]u[η_] := (2*\[Eta]2*η - 2*\[Eta]^32*η^3 + \[Eta]^4η^4) + \[CapitalLambda]Λ/
6*(\[Eta]η - 3*\[Eta]^23*η^2 + 3*\[Eta]^33*η^3 - \[Eta]^4η^4);
\[Theta]θ = Integrate[u[\[Eta]]*Integrate[u[η]*(1 - u[\[Eta]]u[η]), {\[Eta]η, 0, 1}] // N;
\[Delta]δ = 1/\[Theta];θ;
u[y_] := Piecewise[{{1,
y > \[Delta]δ}, {(2*y/\[Delta]δ -
2*(y/\[Delta]δ)^3 + (y/\[Delta]δ)^4) + \[CapitalLambda]Λ/
6*((y/\[Delta]δ) - 3*(y/\[Delta]δ)^2 +
3*(y/\[Delta]δ)^3 - (y/\[Delta]δ)^4), True}}]
Assuming[yd \[Element]∈ Reals, NSolve[Integrate[u[y], {y, 0, yd}], yd]]
{{yd -> 0}, {yd -> 9.01341}}