Players on your roster can be improved with a combination of both currency and XP. XP is collected when trading in unwanted players from your inactive roster or when obtaining a player you already have on your roster.
All players you acquire will be starting from Level 1 at the Bronze tier, so it’s up to you to make them better!
When a player reaches level 10 at his current tier, he cannot be promoted to the next tier unless your team is at the appropriate tier. For example, if you’re at the Bronze tier, a bronze level 10 Jose Altuve won’t be able to move up to Silver level 1 until your team attains Silver tier status.
If you acquire a player already on your roster, your player will automatically level up. For example, if you have a Bronze level 2 Yu Darvish and you get another bronze Yu Darvish, you’ll now have a bronze level 3 Yu Darvish.