Let’s see how we can change the sequence number for Purchases:
Please follow the path: Settings > Custom Fields & Preferences Section.

Follow the following steps:
1) Select the Purchases under the option of “Set Preferences for”.
2) Now you can select any sequence no. option from below three:
– System generated voucher number.
– I’ll input my own voucher number.
– Sequential voucher number.
You can also add Prefix, Starting sequence and suffix.
Click on save and its done.
You can also add custom fields for Purchases., estimates, sales order etc.