You can take up to 25% of your pension as an upfront tax-free lump sum.
You can either take this in one go, moving 75% of your pension to a drawdown fund. This 75% will be taxable as income when you take it. Alternatively, you can opt to move your pension to drawdown gradually, each time taking a proportion of your available tax-free cash with a sum equivalent to three times the tax-free cash moved to a drawdown fund.
A few people in older pension arrangements may have a protected right to a larger lump sum than this — if this applies to you, it should be outlined in your pension documents.
Failing that, an adviser who regularly deals with pensions will be able to discern whether this applies to you.