I used the replace rule, but it didn't work. Would you like to show how to use the replace rule correctly?
In[400]:= eq33 = eq32 /. {A1 -> Sqrt[Dc]*F1* \[Lambda]}
Out[400]= \!\(
\*SubsuperscriptBox[\(\[Integral]\), \(0\), \(T\)]\(
\*FractionBox[\(
\*SuperscriptBox[\(Dc\), \(3/2\)]\
\*SuperscriptBox[\(E\), \(\(-
\*FractionBox[
SuperscriptBox[\((lh - x)\), \(2\)], \(4\ Dc\ t\)]\) +
\*FractionBox[\(\(-Dc\)\ t\ \[Theta]c + Dp\ T\ \[Theta]c\), \(Dc -
Dp\)]\)]\ F1\ \[Lambda]\), \(\((Dc - Dp)\)\
\*SqrtBox[\(\[Pi]\)]\
\*SqrtBox[\(t\)]\)] \[DifferentialD]t\)\)
eq34 = eq33 //. {(Dp*\[Theta]c)/(Dc - Dp) -> A3, (Dc*\[Theta]c)/(
Dc - Dp) -> A5, -((lh - x)^2/(4 Dp t)) -> -(A10^2/t), (
Dc^(3/2) F1 \[Lambda])/((Dc - Dp) Sqrt[\[Pi]] ) -> A11}
Out[409]= \!\(
\*SubsuperscriptBox[\(\[Integral]\), \(0\), \(T\)]\(
\*FractionBox[\(A11\
\*SuperscriptBox[\(E\), \(\(-
\*FractionBox[
SuperscriptBox[\((lh - x)\), \(2\)], \(4\ Dc\ t\)]\) +
\*FractionBox[\(\(-Dc\)\ t\ \[Theta]c + Dp\ T\ \[Theta]c\), \(Dc -
Dp\)]\)]\),
SqrtBox[\(t\)]] \[DifferentialD]t\)\)