Which drug should I replace with inderal (10mg) to prevent baldness?

There are multiple drugs available for treating hypertension in a young male. So, the best thing will be to go to your doctor and ask him to change the same. At the same time, it is important to share your family history of baldness also with the doctor as male pattern baldness can start appearing in this age irrespective of the medication you are taking...

Beta blockers can cause hair loss in some users. Of course, I can't determine without knowing more about your story and seeing your scalp up close if it's related in your case or not.  If it is - there are many many other types and classes of blood pressure medications that can be used. Be sure to review with your physician. Most people who use beta-blockers such as Inderal do not experience hair loss. Make sure you determine carefully if you are experiencing a drug induced telogen effluvium or whether you have early onset androgenetic loss. 

Ask your doctor about spironolactone, minoxidi (oral), and many of the other antihypertensives out there.  Many of them will both control blood pressure AND help hair loss!  At 21 years old you should also consider finasteride.  It is very likely the inderal is not the sole cause of your hair loss - genetics and DHT are!
Good luck!