H is abelian, because for any $\displaystyle a,b \in H: \ 1=(ab)^2=abab,$ which gives us $\displaystyle ba=a^{-1}b^{-1}=ab.$ thus every element of H is in the form $\displaystyle h_1^{k_1} h_2^{k_2} \cdots \cdots h_n^{k_n},$ where $\displaystyle k_j \in \{ 0,1 \}.$
to finish the proof, you only need to show that such a presentation for an element of H is unique. this is a result of the set $\displaystyle \{h_1, \cdots , h_n \}$ being a minimal set of generators:
if $\displaystyle h_1^{k_1} h_2^{k_2} \cdots \cdots h_n^{k_n}=h_1^{j_1} h_2^{j_2} \cdots \cdots h_n^{j_n}$ and say $\displaystyle j_r=0, \ k_r=1,$ then $\displaystyle h_r \in <\{h_i: \ i \neq r \}>$ and so $\displaystyle \{h_i: \ i \neq r \}$ would be a smaller set of generators for H. contradintion!