If you're ready to change out your gutters for a bigger size or a different material, you should be able to do it yourself. You may need a helper when it comes time to hang the new gutters, because assembling them on the ground is easiest and the length can get unwieldy on a ladder. The prep work shouldn't be skimped, because the work you do when planning the new gutters will pay off when it comes time to install them.
Step 1 - Plan the Gutters
Since you're changing out old for new, you already have the skeleton of your plan ready-made for you. Measure the existing gutters and downspouts to get the correct length. Note how many elbows you'll need and how many hangers are used to hold the gutters in place.
Most gutters are either 5" or 6" gutters. If you're changing out to a bigger gutter you only need make sure that the side that mounts is about the same size as your current gutters in order to use the old gutter as a guide.
Step 2 - Test Old Gutters for Slope
Use a level to make sure that the existing gutter slopes down toward the downspout. If it does, you can move on the next step. If it doesn't then you'll need to wait until the gutter is removed to make your installation guideline.
Step 3 - Make an Installation Guideline
Have a helper hold one end of string at the end of a gutter you'll be replacing while you hold the other end of string at the opposite end of the gutter. Snap a chalk line the length of the gutter against the fascia where it's mounted. This will serve as a guide so that you install the new gutter at the right angle.
If your gutter is more than 35' feet long and has two downspouts, you'll need to hold the string in the center at the highest point and snap a chalk line to each end. If the gutter doesn't slope properly you'll need to wait until the gutter is removed. You can use a level to give your line the proper slope and then snap a chalk line as a guide, or you can simply check for slope as you're hanging the new gutter.
Step 4 - Remove the Downspouts
First you'll need to remove the downspouts from the gutter. They should either be attached with screws or rivets, which will probably need to be drilled out. Once the screws or rivets are removed, remove the downspouts. Then remove the metal clamps that hold them in place against the house.
Step 5 - Remove the Gutter
You'll either have to unscrew the fasteners holding the gutter in place or use a hammer or pry bar to remove the nails that hold the hangers in place. You may need help holding the gutter in place while you remove all the fasteners to keep it from scraping the fascia or falling off the building.
Step 6 - Hang New Gutters
Use your chalk line and the spacing of the fasteners as a guide to hang the new gutter. Connect the downspout, and you've successfully changed out your old gutters for new.