In this reaction ${1^o}$ carbonium ion is formed which rearranges to form ${3^o}$ carbonium ion from which base obstruct proton. Hence $2-$ methyl-$2$ -butene is formed as a main product.

$\mathop {\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,C{H_3}} \\ 
  | \\ 
  {C{H_3} - C - \mathop C\limits^ +  {H_2}} \\ 
  {|\,} \\ 
  {\,\,\,\,\,\,C{H_{3\,\,}}} 
\end{array}}\limits_{{1^o}\,carbonium\,less\,stable} $ $\xrightarrow{{Methyl\,shift}}\begin{array}{*{20}{c}}
  {C{H_{3\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {\mathop {C{H_3} - C - C{H_2} - C{H_3}}\limits_ +  } 
\end{array}$

 $\xrightarrow{{Elimination{\kern 1pt} of{\kern 1pt} proton{\kern 1pt} from{\kern 1pt} \beta {\kern 1pt} carbon{\kern 1pt} which{\kern 1pt} is{\kern 1pt} less{\kern 1pt} hydrogenated}}\mathop {\begin{array}{*{20}{c}}
  {\,C{H_{3\,\,\,\,\,\,\,\,\,\,\,\,\,\,\;}}} \\ 
  {\,\,\,\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {C{H_3} - C = CH - C{H_3}} 
\end{array}}\limits_{2 - Methyl - 2 - Butene} $