Daniel is correct... But, if you are new to Eagle, here are the complete steps...
- Find the "Wrench" symbol, in the right-side tool list. This is the Change Function. Click on it.
- Scroll to the bottom of its drop-down list and find the "Width" option.
- Click on that and scroll down to the width you would like to use.
- Click on that width.
- The cursor is now loaded to change whatever tracts you tap on with the new width.
- Tap on the track.
- You may have to click on the rats nest to update the screen.
If you are just starting to learn Eagle, you may want to read the recent news about how it will be offered/sold, going forward. The new owner is going to subscription based pricing. Lots of folks are not pleased about that. You might rather invest your learning effort in something else. KiCad is a free, open source alternative.