To delegate you go to https://app.uniswap.org/#/vote and hit the delegation button once you have done so another prompt will present you with the options to self delegate or to delegate to someone of your choosing
Once you have clicked on Add Delegate + you then will input the ETH address of your delegate.
For example if you wanted to delegate to myself you would place 0x15a49309a1d61436dfA67F6dE07ee0235dEBe8Bf in the section Recipient.
When satisfied with your decision click on Delegate Votes to complete the process.
To add to your delegation you simply increase your UNI holdings. I found this out when I bought some UNI then went to increase my voting power and noted that the system had done it for me.
As far as removing delegation it seems that there are two ways to go about it do I am only making assumptions.
You simply delegate to someone else or move the UNI to another wallet.
The UNIs seem currently spendable do I might not be correct in this regard. It is not locked in Uniswap such as when liquidity is provided.