The easiest way would be to repeal the laws that forbid voluntary full metrication and trade in metric only products.
For example, the FPLA is a Federal law covers packaged goods that you see in the supermarket. It mandates non-metric units on the label. A two liter bottle with a label that says "2 L" is legal in all countries except the USA. There is a proposal to change the FPLA. Many manufacturers want the freedom to offer metric-only labels (as you can see with wine and liquor) and don't want to pay the costs of relabeling just for the US.
Many Americans are unaware that non-metric units are mandatory.