Hi Mohammad,
The VLAN will change from the default to the VLAN you need automatically, you don't need to remove it first.
For example, if you created a new VLAN 100... and your router is on port 1/1... and you want it untagged...
configure vlan 100 add ports 1/1 untagged
or if tagged
configure vlan 100 add ports 1/1 tagged
Regards,
Rob