Can I change my will at a later date, if I change my mind?

If you want to make a minor change, you can do it by adding a ‘codicil’ (a separate document which must be signed and witnessed like a will and is kept with your original will). For instance, you might want to leave a fine mirror to a friend or helper: that would make a suitable subject for a codicil. Codicils have to be signed and witnessed like the original will (though the witnesses can be different).
If you want to make more significant changes, however – perhaps because of the death of one of the original beneficiaries, or the birth of a new grandchild – you should draw up a new will.
What you should avoid doing, under any circumstances, is writing on the original will. That can create problems and might invalidate the will completely.