Question

how can u change a sigma bond into a pie bond

Answer 1

Recall that a sigma bond is a single bond where two orbitals have significant overlap. For alkanes (hydrocarbons with single bonds only) all carbon atoms have an #sp^3# orbital hybridization. The #sp^3# arrangement creates 4 hybridized orbitals that are #109.5^@# apart from one another with a tetrahedral geometry.
Consider Ethane (#CH3CH3#)

Ethane has 4 sigma bonds for each carbon (one to the other carbon, three to each hydrogen). With the
s
p
3
hybridization, there is no way to a double bond between the carbons without causing significant strain and interference.
Consider the orbitals for ethene (
C
H
2
C
H
2
), an alkene with a double bond between the two carbons.