There are numerous moneychangers on Changspa Rd, Upper Fort Rd and around Main Bazaar. Paul Merchant gives reliably fair rates for cash and travellers' cheques and is far less bureaucratic than using J&K Bank, where rates seem generally no better.
ATMs are increasingly widespread, with many in the Main Bazaar area. If using a non-Indian card, the charges and exchange rate that you will eventually be charged can vary considerably between ATMs: we found the SBI to be fair. On rare occasions almost all Leh's ATMs have been known to temporarily stop functioning; it's wise to keep a stock of cash available.