If you've received a bonus there are a couple places and ways you can claim them.
1.1 Hover over your profile
1.2 Claim your bonus from here
If you only have one bonus sitting waiting to be claimed, you can claim it directly from this popup by clicking "Claim" on it.
2.1 Hover over your profile again
If you are looking to claim a different bonus, for example if you have multiple waiting, you will need to navigate to the bonuses page. You can do so by hovering over your profile again, but this time selecting the "Bonuses" page.
2.2 Claim your bonus
On this page you can view all the available bonuses for you to claim, and you can claim it by simply clicking "Claim". This will instantly apply it to your account.
If you have claimed a Profile Avatar, you can enable it by following this guide here:
If you try to claim a bonus multiplier whilst you have one already active, it will replace your existing bonus rather than stacking. You will be asked if you are sure when trying to do this.
If you have claimed a game key or reward as your bonus, you can find it by navigating to your Reward Vault.