My MacBook Pro says service battery on menu bar how can i fix it?

About Mac notebook batteries:

• Service Battery: The battery isn’t functioning normally. You can safely use your Mac when it's connected to an appropriate power adapter, but you should take it to an Apple Store or Apple-authorized service provider as soon as possible.

About Mac notebook batteries - Apple Support

Contact Apple.

  https://www.apple.com/retail/geniusbar

 If you still need help, contact Apple Support, an Apple Store, or an Apple Authorized Service Provider.

Replace the Battery.

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"—

https://www.apple.com/retail/geniusbar/

You do not say what exact Mac this is— older Macs you can replace the battery yourself, search:

DIY Battery Replacements for Apple MacBook Pro ... - OWC