Changing existing with current will not change anything here.. except for one thing. "Existing" does not imply a change of machinery is ever going to happen. It might. But the word just means "this is here."
The word "current" suggests (that their may have been other machines in the past, their may be other machines replacing it in the future. This machine is one in a timeline of machines that will be here. This is just for now.)
Fix and modify are different in an important way. Choose the word that fits the best description. It makes a difference.
Fix is used when there is a broken machine that is repaired. The fix brings a non-working machine to the state of working.
Modify is used when something is brought from a less useful (possibly useless) condition to a better or more useful condition.
Have a great day.