There're two ways to change files encoding, you can change them locally or globally.
Change file encoding locally.
Please right-click on any folder or file in Project Explorer. Then click "Properties" on the menu.

Eclipse Change File Encoding - 01
Change "Text file encoding" as you like.

Eclipse Change File Encoding - 02
Change file encoding globally.
This option will apply all files in the workspace. First, click Window > Preferences

Eclipse Change File Encoding - 03
Expand General > Workspace, then change "Text file encoding" as you like.

Eclipse Change File Encoding - 04