If you have a solution with 0.5 U/mg is the same of 500 U/g, so if you wish 50 U/g you must dilute 10 times that solution in the appropriate solvent or activity buffer.
Do the 50 U/g refer to the concentration (activity per weight of solution, which would be unusual as the concentration is normally given per volume), or does it refer to the specific activity (U per g of total protein, irrespective in which volume the protein is dissolved)? In the former case, you'd dilute with buffer, in the latter with the solution of an innocuous carrier protein (usually serum albumin, which is relatively cheap).
Btw., the enzyme unit (1 U = 1 µmol/min) is an outdated unit of activity, in the SI the correct way to express activities is in katal (1 kat = 1 mol/s, 1 U = 16.7 nkat).