Complex sentence: IPP is formed from acetyl-CoA via the mevolonate pathway (the "upstream" part), and then is isomerized to dimethylallyl pyrophosphate by the enzyme isopentenyl pyrophosphate isomerase.
Split into simpler sentences: IPP is formed from acetyl-CoA via mevalonic acid pathway. IPP can then be isomerized to dimethylallyl pyrophosphate by the enzyme isopentenyl pyrophosphate isomerase.