You can't always, not all companies allow early exercise.
If you've been at a private company for a while you may have options that have vested approaching the ten year expiration with a large spread between strike and the fair market value.
If these options are ISOs then exercise has a lot of tax consequences to get right (AMT particularly to save some money). If you have NSOs you've still got a lot of tax to deal with on exercise, but you don't have to benefit of getting some tax free below AMT.
If you've been at a private company for a while you may have options that have vested approaching the ten year expiration with a large spread between strike and the fair market value.
If these options are ISOs then exercise has a lot of tax consequences to get right (AMT particularly to save some money). If you have NSOs you've still got a lot of tax to deal with on exercise, but you don't have to benefit of getting some tax free below AMT.