Open Gmail
Click the gear icon
Select See all settings
Go to the Labels tab
Find the label you want to remove
Click Remove under the label
Confirm the removal if prompted
To remove a label from a specific email, open the email
Click the Labels icon
Uncheck the label
Click Apply
To delete a custom label completely, hover over the label in the left sidebar
Click the three-dot menu next to the label
Select Remove label
Confirm the action if prompted