How do I make the currency selection field editable for leads and opportunities?

To allow salespeople select a different currency for leads and opportunities, you must make the Currency field editable. In addition, set up multiple currencies for sales by enabling the Enable Multiple Currencies for Opportunity Revenue Lines multicurrency setup profile option.

By default, the currency selection field isn't editable for leads and opportunities. Here's how to make the field editable:

Ensure you're working in an active sandbox.
Navigate to Configuration > Application Composer.
In the navigation tree, expand Standard Objects, expand Opportunity or Sales Leads, and click Pages.
In the region that corresponds to the page you're changing, duplicate the standard layout and edit the resulting layout. For example, if you're making the field editable in opportunities, in the Creation Page Layout region, duplicate the standard layout and then edit the resulting layout.
Select the newly created layout, click Actions and then click Edit UI Properties from the drop-down list.
Select Currency from the Field Selection drop-down list.
Select the custom layout in the Layout Properties section.
Click Update.
Select the Updatable option.
Click Submit.
Click Done on the Edit UI Properties: Creation Layout window and save your changes.
While still in the active sandbox, navigate to Sales > Opportunities or Sales > Leads as a user who's a sales resource, such as a sales administrator or salesperson. Verify the change by editing an opportunity or lead and validating that the Currency field is a drop-down list that users can select.
Publish the sandbox according to your company's business practices.

To set the profile option to enable multiple currencies on product lines, see How can I view the list of currencies I've enabled in the Currency field for my Opportunity?