Ok. Now I don't know much about this, but I wanted to change my current substrate, small pebbles, to sand.
How can I go about this??

Done this myself a while ago. But a bit of work and a real pain to be honest.

What I done was remove the gravel in stages over a couple of weeks during my normal water change / cleaning. Depends on how much gravel you have in your tank as well.

You must amke sure you clean the sand well before putting any in your tank as it will go very cloudy. Working outside I half filled a bucket with sand and put a hosepipe in it. I kept moving the sand with my hand or a stick until the water on surface is clear. This can take a while until its done. Friend used a decorators plaster/paint stirrer in an electric hand drill to stir the sand whilts flushing with a hose.

When the sand is cleaned I put it into a plastic bowl that I lowered to bottom of tank and then gently poured out onto bottom of tank.

A good way to remove the gravel without distrubing it to much was to use a suction gravel vac. Depending on gravel size it will suck it up into the hoover. When you have a good amount in it lift it to surface and place your hand over the end and empty out into a bucket.

i did this the other day and ill tell u its not worth the effort.

i used a vac and sucked the gravel out bit by bit during my weekly water changes

Hm, had I read the replies in this thread before, I'd probably not have done that ever, lol. The few times I did, I just took the gravel out with the help of a syphon, waited a bit for the dirt to settle again, then sucked it from the bottom and put in the cleaned sand bit by bit. Didn't take long at all, just forget about nice fingernails when digging in gravel
Seeing the dirt that still collects in/under the gravel is one reason for me to prefer sand. The tanks were a bit cloudy from the sand for a day or so, but not too bad.

I recently put an under gravel filter in my tank (put an outlet from an external filter to push water thro the UGF- keeps the gravel very clean so no more gravel cleaning). I took all decorations out and them used a scoop to pick the gravel out. The worst part was getting the last few bits out. It didnt take as long as i thought. About an hour to 1.5 hours

so what kind of sand can you use.
is normal builders sand fine. (yellow sand)
we are self employed building firm so i could get this for free as theres 9 ton of it at the job i just wasnt sure if i could use it for my tank.

when i changed my tank over from gravel to sand i simply took out all the gravel and very slowly added the sand into the tank.
my water was cloudy for about 2 days but the filter took care of the floating stuff. i used regular mortar sand (used for laying bricks and blocks) kinda like beach sand but not quite as fine. picked it up from a job site at no charge. its been in the tank for about 4 months now and i havent had any probs at all with it.

cool yeah same stuff i can get yellow sand. do you have any pictures of it in your tank as when its wet it goes realy dark and vibrant

whats it like to clean do you just use the gravel vac still for the surface mess as i know it wont burrow down like it does in gravel

the tank im builing should be done in about a week(all i have left to do is set the glass) when its done i'll post pics of both my tanks(one oscar tank and one african cichlid tank)

nice one im in the process of tanking away my gravel this week bit by bit going to be a fun week haha