Memory in phones and tablets is soldered directly on the motherboard. Unless you have access to a surface-mount reworking station to desolder the existing chips, clean the solder pad, buy higher-capacity chips with matching footprints and pinouts, put solder paste back on the pads, solder-reflow the new chips in place and then update the OS with new memory limits - assuming all the necessary traces to support the extra capacity are in place - you can forget about it. Unless you are an experienced SMD rework tech with access to all the necessary equipment and supplies, you will most likely kill your phone.