If yours is an older home, you may have galvanized steel water pipes in the walls, and if you do, your water system may not be functioning optimally. Galvanized pipes have a tendency to corrode from inside, and as the rust builds, it also restricts water flow. The result is that you have less water pressure at the point of usage than you need to have. If you have isolated some issue pipes behind the sink, it is not difficult to replace them with copper. You just have to put in a dielectric union at every joint where the dissimilar metals match to prevent corrosion.
Shut the water off to the sink. The most expedient way to do this is usually to close off the main water valve to your home.
Unscrew the water supply hoses from the faucet with adjustable pliers and disconnect the P-trap by unscrewing the nuts which connect it to the sink tailpiece and the drain. If you can, disconnect the sink from the wall and move it out of the way.
Make a hole in the wall large enough to expose the pipes that you want to replace. If the wall is covered only with drywall, cut a bigger hole than you need with a masonry saw. It will not be any more difficult to repair than a smaller hole and will make replacing the pipes simpler.
Cut out the part of pipe that you want to replace using a hacksaw. Make 1 cut 2 inches from the fitting at one end of the department and another cut the exact same distance from the fitting at the other end. Pull out the department and unscrew the cut tubes from the fittings using a pipe wrench.
Replace each of the pipes you eliminate using a 4- to 6-inch Nail nipple. Wrap pipes tape to the threads of this nipple, then tighten it as much as you can by hand and use a pipe wrench to tighten it the rest of the way.
Expand the galvanized half of a dielectric marriage onto each nipple. Leave the marriages assembled as you dry-fit a method of copper pipes and fittings involving them to replace the galvanized pipes you eliminated.
Disassemble the marriages and solder all the joints in the pipe meeting using a torch and lead-free solder. When the pipes have cooled, then assemble the marriages again and tighten the central nuts using a pipe wrench.
Turn on the water and check for leaks before you patch the wall, using appropriate materials, and replace the sink. Hook up the P-trap and connect the faucet supply seams to fill out the repair.