dtb is a (compiled) dts. In the post above it is suggested to replace the dtb with a modified one, indicating the dts has been altered.
if you have terminal access you can unpack the dtb using the dtc.
As far as the modification,I believe its pretty straight forward. You need to increase the rootfs partition to utilize the extra space
ie
partition@800000 {
label = "rootfs";
reg = <0x00800000 0x7800000>; <--- =+/-7000000 hex
};
to
partition@800000 {
label = "rootfs";
reg = <0x00800000 0xF000000>; <--- +/-E800000 hex (double sized)
};
keep in mind this is an example, the idea is you increase the original offset (0x7800000 in this case) to suite your needs. Depending on the layout and where the roofs partition is located, it may be necessary to adjust each partition after the rootfs partition to reflect the changes. In the example case, its very easy because rootfs is the last partition, so increasing the size doesn't affect any other partition.