ha ha ha... for which analyte you are trying this conversion?

First you must know the gmw or gram-mole-weight of that analyte.... for the solution of, say, calcium, the gmw is 40 which means 1 mole of Calcium is 40 g. It also means 1 millimole (i.e., 1 mmol)  = 40 mg of Ca.

Now, say you have a solution of 1 mmol/L, let's convert it to % concentration:

1 mmol/L = 40 mg/L of Ca solution = 0.004 % Ca solution. (Because 10,000 mg/L = 1%)

So the formula for you is as follows:

1 mmol/L = (gmw/10,000) %

2 mmol/L = 2 * (gmw/10,000) %  and so on.

So, multiple your whatever mmol/L value by (gmw/10,000) to obtain in % concentration.