First I have designed the network its showed more no of pipes not meeting minimum velocity constraints, cover constraints and slope . After changed to analysis some pipe diameter and slopes are changed but still out of 171 pipes nearly 71 pipes not meet the minimum velocity constraints
When you are working with small sewers with only a few customers upstream, you will almost always have many pipes that don't continually meet the minimum velocity constraint. When you get into these low flow situations, you need to remember that for these pipes, flow is not steady but comes in pulses when a customer drains a washing machine or dishwasher, flushes a toilet, runs a shower,... During these times, those pulses usually provide sufficient tractive force to move any solids.
Those minimum velocity constraints really don't apply in these situations. All you can do is meet the minimum slope requirements as given in you local regulations or in publications like the ASCE Gravity Sewer manual. If you meet minimum slope, you are probably safe.
If the area where you are working is almost completely flat, you may have trouble meeting even minimum slope requirements. In that case, you may need to consider pressure sewers or multiple pump stations. These are more expensive and difficult to maintain, so you'd like to use gravity sewers.