Consider removing the armrests if possible, or getting another chair without armrests or with armrests which can be lowered out of the way.

Some chairs are constructed so that you may be able to attach wooden blocks between the armrests and the bottom of the chair, which would effectively lower them.

You can try using a seat cushion, but it is recommended that you do not use a cushion for more than an inch of elevation, because it can be unstable. If you choose to use a cushion, you must treat the cushion as if it were part of the chair seat for the remainder of the adjustments in this guide.