Do you want to limit the bandwidth usage of a cPanel user? Or is it the case that your bandwidth is limited and you need to increase it? A WHM user can modify the cPanel account’s bandwidth and a Root user can modify the WHM and cPanel user from their WHM account.
1. Login to your WHM Account.
2. Go to Account Functions >> Limit Bandwidth Usage or type Limit Bandwidth in the search box. Then, the option will appear. 
3. In the Search Box, type the cPanel username or select a user from the drop-down list. Then click on the Limit button. 
4. Under Bandwidth Limit, select the Unlimited radio box for unlimited bandwidth or type the value in the field box and then click the Change button. 
Congratulations. You successfully modified the bandwidth limit of your selected cPanel user.