Regarding to Base chain, How can I un-delegate my tokens?

How can I un-delegate my tokens?
Please refer this link to un-delegate your tokens: https://dashboard.dappchains.com/staking-guide

And how long will it take to regain full access to my tokens?

You will gain access to them at the end of the current election cycle. This is a security measure to mitigate against scenarios of staking with a bad candidate to increase their odds of gaining the right to validate, and then immediately un-delegating.
The election cycle, in simplest terms, is a repeating countdown timer. Each time the timer hits 0, any new state changes — such as delegations, redelegations, or reward payouts — are updated. The election cycle period is currently 30 minutes, and the countdown can be found on the Validators page
For more infomation, please refer: https://dashboard.dappchains.com/faq