Once again you must know what type of tree you have. Some trees grow so fast they must be root pruned and re-potted yearly. Others may need it every other year or some exceptionally slow growers every 3-5 years. Generally, tropicals are best re-potted and root pruned during the hottest part of the year when they are actively growing. Re-pot most temperate climate woody trees just before bud break or when they first start showing fresh white root growth. Usually this is accomplished in the early spring. Some species can also be re-potted in late fall. When re-potting a finished bonsai, the general practice is to remove one half to two thirds of the old soil and prune one third to one half of the roots. The tree is then replaced in the same pot.
You should prune foliage from the tree when you remove roots. Pruning a comparable amount will save stress on the tree. For example, if you remove 1/3 of the roots, top prune 1/3 of the foliage. It may be easier to prune the foliage while the tree is still in its pot. Have everything you need at hand before you begin, tools, soil, pot, screen and wire. Do not do this in the bright sun light or the roots may dry out. Once the root are dried out, the tree will die and all efforts will be lost. Please bear in mind to chose a good spot to do the work, as this is ab absolute fundamental aspect in the process of re-potting.